Interesting. I seem to be the only person in this thread who's playing a non-sneaky non-fighter.
Sierra likes fire. Lots and lots of fire. And not being sneaky at all. Because fire is the anti-sneaky. In fact, if I even appear to be remotely not-seen by nearby monsters, I pop Candlelight to make sure everyone can see me.
So that I can set them all on fire.
Just hit level 10 last night/this morning, pretty much pure Destruction with a dab of Restoration (to get to the regen-magicka-faster perks) and Speech (because after setting everything on fire I end up with a lot of stuff to sell).
Accomplishments thus far: soloing two mammoths and a giant, all at the same time, at level 5, and killing Krosis, who I stumbled onto entirely by accident while checking out compass markers in the mountains en route to Whitehelm, at level 9/10 (I can't remember if I hit 10 before or after the very long fight) using only Fire Rune and whatever that flame-stream novice spell is called. And only died once in each of those fights. Normally being able to kill massive badasses while so extremely low level and without any awesome gear would lead me to say that the game is too easy, but those were some epic-scale kiting cat-and-mouse battles.
This is the most fun I've had with a Bethesda game. Ever.
Sidenote, for anyone planning to/interested in leveling magic skills, if you didn't already know, using higher-tier spells gives you a lot more skill experience. A LOT more. Apparently it also scales off of either how much damage you do with it or how many targets you hit, or both, I'm not sure; all I know is that using double-fisted fire stream on things is pretty lethal but slow skill gain, but landing a Fire Rune (Apprentice spell) on 2-3 targets gives me about 1/3 of a Destruction skill level in one shot, and I'm already up to 48 Destruction. Recommend getting those use-higher-tier-spells-for-half-mana perks quick so you can use the high-xp spells sooner and more often.