Think of it this way: a fighter uses a weapon skill to deal damage, right? So One-Handed, Two-Handed, or Archery. You'll probably have enough perks to dabble in a second skill, whatever, cool. It's fun to play a warrior and it should be. Now, let's say you're using One-Handed. As your skill goes up--whether or not you put perks into the One-Handed tree--your damage with one-handed weapons increases. So your damage constantly goes up whether or not you get new weapons. Of course, you will get new weapons, because you're not an idiot, and they provide a nice leap in damage output every now and again.
Cool!
A mage uses Destruction to deal damage. The equivalent of getting a new weapon for them is getting a new spell. But here's the problem: spell damage does not increase as the Destruction skill does. At all. Not one point. So eventually, after you've gotten all of the available spells, and you're level 40 or higher, your damage will never increase again. Ever. You can't enchant your gear to give you, say, +20% fire damage from spells. You can't rely on your increasing Destruction skill to increase spell damage. Once you have the best Destruction spells in the game, that's it. That's as good as it'll ever get. But enemies will keep getting stronger, and keep having more health.
I know it's easy to say, "Use all of your spell schools!" And that's right, you should. Just like a warrior should use Block, an Armor skill, and probably Smithing to be the best he can be. But a warrior can still rely on his weapon for damage no matter how high his level gets. Destruction is a mage's weapon, but eventually, it can no longer be relied on to do damage.
This is a problem because, without exploiting Alchemy and Enchanting, spells cost magicka. And yes, power attacks and blocking cost stamina, but you can still do something when your stamina runs out. A warrior with no stamina can still use regular attacks while his stamina regenerates. If a mage has no magicka, he has absolutely no recourse. He can run, or he can die. No amount of Conjuration or Illusion spells will help with that, because they cost magicka, too.
Do the complaints make a little more sense now? All we're asking for--literally the only thing--is for Destruction spell damage to get better as Destruction skill gets better. We don't want to be able to use crappy level 1 spells the whole game, but we do want to be able to use the best Destruction spells to deal decent damage after level 40.
Why do people keep insisting that it's not an issue until level 40? It becomes glaringly apparent that it's a problem well before then when you fight anything that's not your standard generic enemy that would fall over from two or three swipes of a sword. It's a problem even on Adept level, and it's disgustingly awful on Master. I'm not saying you don't understand the issue, only that it's not just a problem once you're reaching the end of the game. Well before that.
-Until/Unless you exploit Enchanting, mana will always be a problem because the +%regen enchants do absolutely nothing during combat. Many fights require you to kite or run away waiting for mana or drinking lots of potions
-Impact is not the be all, end all solution that all of these "Destruction is fine, L2P" harpies would suggest. On top of that, it requires the Dual Casting perk...which is a HORRIBLY INEFFICIENT perk that drains your mana even faster than before with almost no real extra benefit. It's entirely worthless until you even get Impact. The game also does not consist of a single file line where you only have to worry about a single enemy at any given moment.
-You actually have to spend so many levels on Magicka that both your health and stamina will suffer greatly. Having Stoneskin up without Mage Armor 3 is not the solution either. You'll still die in a hit or two, especially above Adept level.
-Conjuration is the only thing that truly makes Destruction "Sorta, but not quite viable". The issue is that Conjuration is just so strong by itself that you may as well just watch your pets kill everything for you...because they can. And they are infinitely more cost effective than casting fireballs.
I am currently playing on Master difficulty as a "Mage" who mostly uses sneak and a bound bow. I refuse to use pets except in the situations where I keep getting one-shot again and again and don't want to spend 5-10 minutes on a single fight (I am not suggesting physical classes have it hard...). I purposefully limit myself to this, because otherwise the game literally becomes trivial even on Master. If I summon an Ice Atronach, I can clear the room very quickly with my bow (yes, it's MY shots, not the pet). Later on I could summon a Dremora to clear out everything for me if I wanted, but I will simply save it for the rooms where there are NPCs that can one-shot you with their DESTRUCTION spells that seem to deal 6x as much as what yours can. And for tanking dragons just because ducking and sitting every other shot gets monotonous.