Bethesda have done an amazing job here - their ambition since Elder Scrolls I has been to create a totally free environment, and this time (IMO) they're really achieved it.
I'm a big RPG fan, and I love the way you can follow somehing like 6 or so totally different career paths all at the same time, or in any order you like, plus you can explore the world in any direction at any time. It's beautifully non-linear. Often you head off on a quest, then get totally lost (instructions on getting to dungeons and hideaways are realistically vague and take some effort to follow). You end up stumbling upon a random den of cut-throats and come away a little while later with heavier pockets and some nicely refined combat skills. It's so easy to get sidetracked.
Levelling has been really well redesigned - you choose your core skills, gain experience in them directly, and when you've progressed them sufficiently you gain an overall experience level. Clever stuff.
The big problem, as most people will tell you, is stability. I haven't had *too* many crashes, but I'm a fairly tolerant gamer, and there're enough bugs to really spoil the experience if you don't save regularly (or just hate crashes).
Graphics are something special, but very power-hungry. I found the environments a bit too empty though. You don't get a massive variety of creatures, and there are very few animated touches, so the world doesn't really come to life.
The one thing I missed in this game, which I realise was not on Bethesda's priority list, was a 'cinematic' main quest. The main quest is good and engaging (so much as I've seen to date), but there are no cutscenes, animated sequences, or multi-character conversations, so it all remains rather impersonal and nobody ever comes to life. All you get is text conversations and solo adventures. Personally, I prefer a good balance of linearity and scripting with my non-linearity, but this has been so well done I couldn't knock it under 90% and live with my conscience!