Wonderful post, OP, and I think I can help your situation. Forgive me if this is short, as my girlfriend is looking for a movie for us to watch (in which case I'll continue tomorrow--I really want to contribute to this!).
This is meant more as an attempt for those who play as mages to explain how their characters work, what I am going to say has just been my experiences so far.
I have wanted to try playing a mage but I have a very hard time with it because they seem so less effective than my warrior classes in the past. I primarily used Heavy Armor, Shield, Sword and of course maxed smithing. My character was insanely powerful and I found enemy mages to be the easiest to kill. With my elemental protection I shrugged off the half of their attacks, combined with the Lord stone it was even more unfair for them. I never had to worry about running out of stamina because I did a lot of basic attacks and didnt go bashing everything under the sun. Between my armor and my shield I rarely ever needed to heal and just as an added benefit I used Become Ethereal if I was about to get nuked.
My problem is that every attempt I have made at a mage feels so lack luster. His damage is ok sure but he has terrible defense even with Oakflesh. Conjuration seems like a nice distraction tool but my summons die quickly. I think the biggest issue overall is that my magic drains insanely fast in anything above a skirmish. I have points in destruction and the first couple of perks for Novice and Apprentice perks, additionally I put everything into magicka when I level and am wearing fancy robes.
At the end of the day I just dont see the point of playing as a mage except for roleplaying sense. I would love to hear from those who have mastered being a mage to hear what they do to make the style work, suggestions to make myself a better mage, and anything else that you might find helpful. Sorry about the long post.
Your problem is that you're changing your innate playstyle. I had the same issue. Let me explain.
Your warrior, obviously, is very defensive: you use shields and the Lord stone, pour lots of points into health, Become Ethereal is your go-to shout, and you don't even use a lot of stamina in combat. You're a defensive player, and that's why you're enjoying that. Your mage, on the other hand, is offensive: Destruction (obviously offensive), putting all your points into magicka, and Alteration. Even though Alteration is apparently defensive, it's actually mostly useful for offensive mages who like to get into the fray; defensive mages will find themselves not needing it.
I had a similar problem. I prefer offensive playstyles. I love my Khajiit warrior (who I must have rerolled three or four times since 11/11) who uses light armor and dual-wields. He's not extremely durable, but he sure can run circles around his enemies with his weightless armor and tons of stamina while dealing enormous amounts of damage in 360 degrees around him. I had a Nord two-handed warrior, too, who used heavy armor, but that turned out to be the end of him; I didn't like being so sluggish. I even tried shields, but I can't come to like them. In fact, I think the highest my Block skill has ever been is where it is with my current mage (46 with no perks). I don't think blocking is badly done--quite the opposite, in fact--but the playstyle just can't suit me. I find it terrible.
I made a mage a few weeks ago who was defensive in nature. Just Destruction, Restoration and Alteration, meant to tank hits while he cloaked and runed his way to victory. It was the least fun I'd ever had with a mage, and it was very discouraging. Then I made my current build.
This build is aggressive, but in a controlling fashion, much like my Khajiit who is so agile on the battlefield that he rarely gets hit (I'm no PS3, so no speed mods, just honest light-warrior tactics). I'll share the build, and hopefully help you just like you requested. Mages can be incredibly powerful, more so than any warrior.
The perks on the calculator we all love:
Current build at level 42: http://skyrimcalculator.com/#112125
Goal at level 50: http://skyrimcalculator.com/#112126
The game is too easy as a mage. The Archmage's robes are amazing, as well as the Dragon Priest masks--all extremely powerful together. My current equipment is as follows:
-Archamge's Robes (15% cost reduction to all schools, +50 magicka, and +100% magicka regen rate)
-An enchanted pair of boots simply called "Boots" found in the Archmage's quarters (40% shock resistance)
-Switch between an Amulet of Talos (20% faster cooldown for shouts) and an Amulet of Akatosh (+25% magicka regen rate) as necessary, with the Gauldur Amulet as backup if needed (+30 each to magicka, health and stamina)
-A Ring of Recovery (I think that's what it's called, gives +100% magicka regen rate)
-One of three Dragon Priest masks at all times: Otar (30% fire, frost and shock resistance), Morokei (+100% magicka regen rate), and Nahkriin (+50 magicka, 20% cost reduction to Destruction and Restoration). I use Nahkriin the most.
I don't enchant any equipment (as you can see from my build); I use only Illusion, Conjuration, Destruction and Restoration.
Not that I ever run out of magicka in the first place, but if I do, I've got over 100 pounds of staves and dozens of scrolls to back me up. Among my scrolls are those for Firestorm, Blizzard, Mass Paralysis, Bane of the Undead, Guardian Circle, Conjure Flame/Storm Atronach, Fireball, Paralyze, and several others. I have four(!) staves of Fireball, two staves of Chain Lightning, one staff of Wall of Flames, one staff of Expel Daedra (can banish any enemy-summoned creature), and I just got Wabbajack. I've got SoulTrap and Azura's Star (white) to keep those filled, so I can pretty much attack with either magic or my staves with impunity.
I've got too many spells to list, but needless to say, I am unfairly strong. A simple Aura Whisper will tell me where everyone is, and from there they are mine to toy with. Literally every creature that can be detected at any range with Aura Whisper can be affected with the master level Illusion spells, so I like to clear dungeons with Mayhem. There is, at most, only ever one enemy left alive in any given cell at the end of that (and I can cast it several times if for some reason there's still baddies fighting each other after a minute), who I can easily dispatch with my Destruction spells. Enemies can be destroyed before I even reach the room they're in.
Other times, I like to use just Destruction. I felt like Dumbledore when I fouhgt Rahgot. I started out by dualcasting a Fire Rune at the entry (always have a contingency plan--a 120 Fire Rune is a great one), and then I let my magicka regenerate for a few seconds before I entered the room.
In my left hand, a Flame Cloak spell, and in my right, Wall of Flames. As the priest resurrected from his sarcophagus, I embodied myself in magefire and set the room ablaze, then switched to a pair of Fireball spells, healing as necessary. The battle lasted an excruciating minute, with draugr felled from the literal heat of battle for merely approaching me and Rahgot putting up a good fight.
There's hardly a tight spot for me to worry about, so I summon my atronachs for fun and profit, or if I just want to hasten a large-scale battle. Undead are weak against fire, but the stronger ones use frost magics, so either the Flame or Frost atronachs are good choices against them--and since I'm a master conjurer, sometimes both. Usually, though, I summon two of only one type of atronach so that I can use that element of Destruction magic with impunity (Flame atronachs don't mind me setting the entire room on fire). Against mages, dragons, enemies with multiple resistances, or when using Storm Call, I like to throw in a pair or Storm Atronachs. Lots of people argue in favor of the Dremora Lord's power, but while they certainly have it, they are killed too easily, either by the enemy or my AOE spells, so I prefer the elemental giants.
From an early stage, Restoration allowed me to heal almost all of my health at once. Fast Healing confers 75 points, which is useful for a young, squishy mage. Soon I found myself with the 150 point Close Wounds, and now with the 300 point Grand Healing that can heal my allies (and I only have 250 base health. High for a mage, I know, but more on that later). Beyond that, I can ward against enemy magic and dragonfire, repel the undead (but at this point, Illusion can do that), set them on fire and make them flee with Bane of the Undead (which I also have multiple other options for, obviously), and my personal favorite, Guardian Circle: it reserves an entire chunk of the battlefield for my Archmage, who heals 20 points per
second while inside of it, and undead cannot enter it. Guardian Circle is a true lifesaver if I need it.
Altogether, I've only described my use of two shouts: Storm Call and Aura Whisper. Just like you, I can become completely invincible with Become Ethereal, except I can charge a master level Destruction spell while inside of it; one swing of your sword and you're vulnerable again. I have other ways to charge my master spells with impunity, so I honestly rarely use Become Ethereal. I didn't use Slow Time much, but it's getting fun, especially since I have two words now. Fus Ro Dah has obvious utility, as well as Fire Breath and Frost Breath (if I know an enemy's elemental weakness, those two shouts are its undoing). Ice Form, unfortunately, is something I only use for fun, but if I need to escape, I can use that. I can rip an enemy's weapon from out of his hand with Disarm (though unfortunately one of the words is quest-locked through the Thieves Guild), but I have never had a chance to do so, since enemies tend to die. I don't use a lot of shouts other than for fun, honestly.
My mage is an Altmer with
250 base health and everything else in magicka, amounting to 420 base magicka, 470 with the robes and
520 with Nahkrrin, the mask I use the most (well, really 370 base magicka, if you want to get technical, since Altmer have a permanent +50 buff).
Got Necromage, though not a vampire and I don't want to be, at least with this character. Lord Stone, usually, though I used the Steed once and may go back to it again, since I don't have any points in stamina and usually don't have companions (spells tend to kill them and I have summons). Given the aggresive style of build and the fact that I'm rarely hit for damage, the Apprentice is a perfectly useful stone for me. It's going to be a lot more viable when I get the Avoid Death perk, as those 250 points of instant healing will circumvent my magic weakness at least once daily (not to mention Otar's mask). The Lord just suits my playstyle right now, as it helps me shrug off magic and physical attacks just that much easier (my mage is freaking impossible to kill).
Magic regen is 50% from Restoration perks + 100% from the Archmage's Robes + 100% from my ring = 250% bonus regen, and with Morokei, the Amulet of Akatosh and the Apprentice, up to a whopping 475%. I don't really need it that big, though; I've found that 250% is a fine amount, especially given my large magicka pool and cost reduction bonuses. Speaking of which, Conjuration and Illusion cost 15% less to cast (which is perfectly fine with me), while Destruction and Restoration cost 35% less (with Nahkriin)--a good chunk that isn't unbalanced.
With this build at level 42, I can take down a pair of dragons, a dragon priest, and several high-level draugr (deathlords, I'm pretty sure) in mere seconds. My brothers and I have had a good laugh in the awe of this mage. I wasn't kidding when I said that oftentimes enemies have fallen before I even enter the room (and woe if I do!).
This is the
basis of my build. There's a whole lot more to it, but I hope you get the point: the battlefield belongs to me, and it is a courtesy that I allow any to die on it.
I hope I helped!
/I can't wait for difficult expansions *shakes tiny fist*