Test Drive Unlimited 2 was the game I was most disappointed in, though thats not to say its completely bad; on the contrary, I thought it was great, but I expected a slicker experience than the first but unfortunately it was a little messy, especially on the online side of things, which has remained buggy and unpredictable since the launch. Also, I thought the addition of a 'story' and voice acting was terrible and was never needed. I still enjoyed the game immensely, but most things I liked about the first had somehow dissapeared and made for a more generic experience.
I was also dissapointed with New Vegas (which I purchased this year because of mountains of school work around it's launch and Christmas) but again not hugely so. The game was solid, more challenging than F3 (almost ridiculously so with them Deathclaws) but lacked the atmosphere and wonder of the DC wastes of F3. I praise Obsidian for creating a much more 'Fallout-ty' experience and the plethora of factions, reputation paths and choices make for great replay values and more character customisation than F3 (where there was only ever need for one or two characters without mods). Good game, but its lack of atmosphere put me off playing it to the point in which I still have not finished it :sadvaultboy: on the plus side though, the glitches were no different to what I expected from the new Fallouts.
edit: bolded key word, to avoid confusion over title
