I've so far played a good generic version of each mage, fighter, and assassin.
Sneaking untill you get it up really high is somewhat challenging but rewards you with 1 shots or near 1 shots (at this time I can't one shot some of the heavily armored opponents, I can however pickpocket and plant poison to weaken them a bit)
Fighter I'm using a 2 hander greatsword, heavy armor, and doing the smithy enchant (although so far I'm just scratching the smith enchant as I'm just now figuring out how to go bout leveling them up at a decent rate)
Pure Mage so far has proven the most difficult it takes 1 fully wound up double handed firebolt to kill some weak bandits and others I waste my magica pool and cry as I'm running around trying to not get hit and get some magica back. (that I find is the main problem on the tougher enemies you run out of magica and your screwed while neither archery nor melee or whatnot suffer this. Also what makes this worse is the fact that if you hold a charged spell your magica stops regening

)
I think sneaking is balanced pretty well. This is more dependant on AI programming, and is tough to balance, but feels rewarding, and 1 shotting in that regard is fine with me, since in a straight up fight you are not nearly as strong.
Fighter is... broken if you use Smithing or Enchanting. Together it's overkill. Add Alchemy, and you have too muchtime.
Mages really need some love IMO.
Just like its ok to be able to kill the entire population of Balmora in under 10 seconds.

Being an unstoppable badass is part of the fun no? If this guy is upset that hes doing so well, maybe dont grind enchant and smithing? You should be able to do the same thing with spells too if thats what youre getting at.
It is fun being bad ass. But this is too easy, too quickly. Anyone who decides, "Well I want to be a Smith." Will get around half this power. So you'll 4 shot you enemies instead of 2 shotting them.
Spells are not strong enough to do the same thing.