I try to purchase my games are doing research but often get a wee bit overexcited about things and end up buying too impulsively.
Sim City: Except for graphical complexity, I'd suggest Sim City 4 is actually outright better and has a plethora of fantastic modification options for cities. Conversely, the most recent incarnation is bland and full of flaws to me, from the poor traffic system to the teeny tiny size of cities constantly restricting creativity.
Final Fantasy X and X-2: I just really dislike these games and can't really explain it. I found VI, VII, VIII, and IX enchanting experiences that made me cry or squee many times. I had none of that with X or X-2, perhaps it's because the story involvement of Tetsuya Nomura was reduced? I don't know but they felt flat, tacky, and tawdry. This is coming from somebody that actually liked the themes present in Final Fantasy XIII despite the appalling linearity and grating characters.
Star Wars: The Old Republic Collector's Edition: While I enjoyed the first few months and actually really adored the lore provided as part of the edition, I may have been a teeny tiny bit overzealous by getting the Collector's Edition in the first place given the unknown longevity of the game at the time. There are only so many games of Hutt Ball and endless yanking of people across stadiums a healer Sith Sorceress can take before snapping.