Here's a short list, mostly with my disappointments... (Not in Order)
1.Oblivion (pre-mod), Horrible, errors everywhere, I became a vampire and got stuck because I did the main quest first, but it's become fun and addictive because of the mods, which basically beautified it (see Deadly Reflex for combat, Midas Magic for Magic [duh], FranMMMOOO for your sandbox remake and many other mods)
2.Dungeon Siege - UGH! It left a bitter taste...
3.Fable, I didn't hate it, it just failed to meet its expectations, was way to short, made me want more but didn't deliver, and wasn't moddable
4.Dark Messiah of Might and Magic - Actually quite good, until I found out it was 10 minutes long (thats how short it felt), and that it wasn't open a free roaming one.
5.NWN OC - Need I say anything?
6.Beyond Divinity - Divine was good. Beyond, failed to follow.
7.MM9 - Lousy, and failed to live up to the series beauty and glory
8.Fallout 2 - Disappointing, it destroyed the fun times I had with the first game... (Hopefully Bethesda can do good on 3)
9.Diablo 1 - While I did have fun with 2 (my friends and I had great times before we quit out of sheer boredom), 1 was disturbing, not scary, but disturbingly horrible... You've got your Warrior, Wizzard, and Rogue... Non stood out, and it was hacked so quickly that a level 1 character could knock out a level 30 with one punch.
10.Morrowind - Beautiful, until the empty shell phase... Its mods did the same thing that Oblivion's did to me, it kept me hooked, but Oblivion took over when I started getting the good mods...
Mostly just flaws that destroyed some series, I'm looking forward to Fable 2, and Fallout 3, but being cautious not to get to excited and be disappointed.