Day One purchases you immediately regretted

Spore, tried to be too much for too many people. Fun for an intellectual tween, I'd say though I know this is a gross generalisation. 
 
I also love how the regrets thread is three times larger than the pleasant surprises thread.

Spore
Frontlines: Fuel of War (not sure why I got that one...)

Uhh... none that I can think of. If I dislike a game that I bought, I'm still usually in denial about it for the first few days.

Tales of Symphonia: Dawn of the New World. It was nowhere near as good as the first one. 

Pokemon Soulsilver. I'll tell you what though, that Pokewalker made me look like a pimp. 
 
Also, Modern Warfare 2. Bought it, beat campaign first night, then barely played it again.

Socom: Confrontation.  Being a huge Socom 1-3 fan, I was extremely disappointed when I figured out Confrontation is an awful game, especially because I waited an entire year for it with intense anticipation.

Super Street Fighter 4. Got caught up in the hype a second time, didn't like street fighter 4 so you would think I would have learned my lesson. Nope.

Ninja Gaiden II. What in the fuck was I thinking?

 
Bioshock 2. 
I knew from the get-go it wasn't going to live up to the first game. 
It wasn't a bad game, but it didn't evoke the feel of wonder and awe like Bs1 did.

Fable 1 
I beat it later that day and traded it back in. 
 
Edit: Not saying it's bad. I loved playing it, it was just waaaaay too short.

Fable 2 and Prototype are all that come to mind.

Mercenaries 2.

I typically don't buy on day one unless it's a game that I absolutely know I want, like Uncharted 2. And even then I held out for a week and got the limited edition for $60, which I though was a pretty good deal for a limited edition, though I don't know what the original day one MSRP price for the limited edition.

Modern Warfare 2: I don`t know which was worse. The plot of the single player, the balance of the multiplayer, or the grinding I put into the game trying to convince myself that I wasn't overcharged for a game I didn't even like.

battlefield 2142, left 4 dead, only ones i could think of right now. bf 2142 i was expecting it to be more like 2. it didn't feel like it at all. left 4 dead i bought it for the 360. thinking about not playing it anymore i should have bought it for the PC. i bought the second one for the PC. it was a great idea.

Every Steam sale game that I'll never play. Damned weekend deals!

My buddy regretted getting Mortal Kombat vs. DC Universe.

Seeing so many people say FF13 makes me glad I decided to hold back on that and wait for a price drop. A really big price drop. Of course part of this reasoning is because of MYYYYYYYYYY HAAAAAAAAAAAAAAANDS 
Anyway, I have to say Guitar Hero: World Tour(I was expecting better songs... Where's my Crazy On You?) 
Sonic: Er... Dark Chronicles, was it? Well, it wasn't bad. I enjoyed having Cream in my party and all... But that ending, that ending! 
Mario Bik--Kart Wii. Don't care what anyone else says, I hate the fact they added bikes and decided to be "smart" and kept the Mario Kart name. Doesn't feel right. I'll just stick to Double Dash or something. 
Smash Bros. Brawl. I still don't regret getting rid of that game either.

Far Cry 2, and I traded in good games for it!

I remember waiting in line and buying Army of Two (the first one) and regretting it only days later. Last time I relied on my friends' hype for games.

Spore.  The dissonance of what I wanted that game to be and what it was was massive.  An evolution simulator it was not.

Spider-Man 3 for the Wii. To this day, i still dont know why i bought it. PS1 graphics, terrible controls, awful gameplay, i think it's the worst game i have ever played.

Far Cry 2, Got a more fun out of the free T-shirt then the game

This as 2200 views in a day!? Holy shit.
 
I have no gaming regrets.