Rather than posting everything I bought, I'm going to discuss my immediate regrets:
Amnesia
Played about an hour and the atmosphere is as strong as everyone says, but I've always been about gameplay above all else so this sort of game doesn't suit me quite so well.
Audiosurf
Figured it was about time and it was only $1.50. I don't find it fun at all, but maybe I need to try out the different ships a bit more.
Bit.Trip.Runner/Beat
Only played Beat enough to get the first ticket and had my mouse sensitivity set way too high for it to be enjoyable. In general, I don't find it that fun. Runner, I played up to the first boss, but don't feel compelled to play any more. I might beat the first boss and see if world 2 is different enough to respark my interests, but I doubt it.
Magicka Complete
I knew this wasn't a game for me before, but I fell for the slipper slope of "It's only $3" into "It's only $5 more for all the DLC in case I like it." Like with Amnesia, I really should have ignored what everyone else was saying and stayed with what I know: myself.
SpaceChem
I actually like this a lot, but once I got to a puzzle that I knew I could solve but didn't want to think that hard to solve it, I knew I had made a mistake.
Half regrets:
Alpha Protocol, Dragon Age Origins Ultimate, the Last Remnant, Trine
AP and DAOU I've already finished (minus the DA DLC) but the prices were right. Last Remnant I played a good chunk on 360, but I'm not sure if I'll ever get that far in the game again. Trine I bought before on D2D, but basically paid $2 to have it on Steam.
Of course over half my purchases were an hour ago, on old/indie/non-featured games, plus the DoWII:R pricing mistake. I could easily go on to regret most of those.
Not to be overly negative, though, two games have already exceeded their value for me.
Plants vs. Zombies
I figured it was time. Even though I typically don't like tower defense games, the price was right and I had seen enough to risk a couple of bucks. I finished the main game and all the minigames, so I'm probably already done with it, but it was money well spent. Also the music is stuck in my head and I may have developed a slight nerd crush on Laura Shigihara.
Recettear
Steam sale equivalent of a first reply /thread. I was bummed about missing it in the winter sale and was thrilled to see it day one. Not sure how much more I'll play, but I've already sunk over 15 hours into the story + endless mode. I know there's more to explore, but at this point it's mostly 100% this, 100% that type stuff that plagues Japanese developed games. I've never cared to collect 100% of anything just to complete a database and I don't plan on starting now.