I have a mage, warrior and rogue at about lvls 40,50 and 60 respectively. I noticed that as soon as I max out the skill trees to 100 in each of their most used talents (e.g., rogue=one-handed, sneak, light armor, etc.) that my leveling grinds to a halt. I've been at about 15-20% of lvl62 for the last 15 hours of game play --- even though I've hunted dragons, done major quests, run all over the map --- the level bar is barely moving. Does leveling by power-leveling a skill --- like alchemy --- have an effect?

Well, you sort of answered your own question. You will "plateau" around level 50-60 because most if not all of your primary skills will max out so you don't gain XP from them anymore.

At this point to level you have to increase skills that you never bothered with.

So actually doing quests doesn't have as much of an effect as skill leveling?

The ONLY thing that gives you level XP is increasing your skill level.

I actually thought quests don't add any xp to be honest

OMG! Thanks for the info! I think (lol). So, now that I've maxed out all relevant skills to my play style as an archer/assassin (e.g., light armor, archery, sneak, speech, lockpicking, smithing, alchemy, enchanting), I now must level skills like heavy armor, block, two-handed? Or I can become an archer conjurer and use atonarchs as my pet? in other words, I guess no matter how I approached the game I would always have to level somewhat unwanted skills to reach level 80 ... is that true?

Yes.

If you want to continue levelling past your current level, yes. If you don't mind staying the level you are right now then just keep doing what you're doing. Game doesn't depend on levels. ;D

You level faster with skills you use more. Those skills maxed, you have to use ones you use LESS.. this will be a slower process, at first.. but it will take off just as before.

Quests do NOT give xp..

Nothing unlocks over Level 50 in the game. If you have developed your character, now you can just play the game and not worry about leveling. Kind of nice really to not have to grind any more. Any levels over 50 are academic and offer no benefit.

What? You level to 81, you still get a health/stamina/magicka boost every level and a new perk. Right up to level 81.

This is true, but enemies don't scale above level 50, so will just grow more and more overpowered. You don't really need an advantage of 30 levels, do you?
Perks are irrelevant too, unless you want to max out a jack-of-all-trades character. Most specialized builds will be maxed out around level 50 anyway.