Least favourite bit: A tie between fighting Ormstein & Smough, and the bits in the Tomb of the Giants which are pitch black until you find a light/light spell/lit helmet.
Favourite bit: Pretty much all of it.
But particularly the vast, sprawling world it's set in, where (nearly) everything links together. You can get a taste of that from the map viewer someone hacked together:
Oh, and the huge amount of variation - so many different adversaries, each of which behaved in different ways, and needed different tactics to defeat. And I've only played one of the "builds" of the game - Dex-heavy, concentrating on light thrusting weapons. Playing through again with a spell-caster, or a cleric, or even a high-strength club wielder is almost tempting.
Really glad I played it. I think I need a break to do something a little less intense for a while...
*And multiple walkthroughs and FAQs
Original post on Dreamwidth - there are comments there.