Bejewelled 2 is good if you like simple and addicting puzzle games.
Ikaruga and Triggerheart should be no-brainers, unless you've got them for different systems. Same goes for SOTN. Doom is a great FPS; I quite like not having to aim up or down. Keeps things simple.
Aside from Braid the only other game I have is Portal, though that's only because I'm not interested in anything else on The Orange Box, which is a better deal overall - that said, the "Still Alive" extra levels are included with the XBLA version, while you have to DL them separately on the OB.
I've tried a lot of the XBLA games, these are my favorites... which are must tries IMO(of course unless you have another version)
N+ (loots of stuff to play through, and fluid controls. a huge amount of
content for the prices, including the DLC, and the multiplayer is fun)
Rez HD (port of a great game, with sharper graphics)
Ikaruga ( ^ )
Meteos Wars (doesn't look like a lot of people noticed this game, turn up the command speed options and it'll feel better to control, the sound world is amazing and multiplayer is lots of fun if it doesn't lag. Based on the DS meteos game.)
Bionic Commando: rearmed (excellent remake, must have imo)
Galaga Legions (doesnt have much to do with the original, but its fun and manic, with some cool "bosses" and nice graphics and sound)
Castlevania SOTN (pretty good port of a classic)
Megaman 9 (suprisingly, a great new addition to the series)
Triggerheart exelica (well...)
Castle Crashers (nice beat-em-up. got boring pretty quick for me though, and insane mode pretty much needs you to use cheap tactics repeatedly, though its fun with good people)
I ended up getting these:
Geometry Wars 2
Bionic Commando
Mega Man 9
Omega Five
Space Giraffe
Pac Man CE
Street Fighter II HD
Worms
Bomberman Live
Mutant Storm Empire
Thanks for all the suggestions. So far all I've bought are Geometry Wars 2 and Worms; it'll take awhile to sift through all these. If nothing else, GW2 will keep me busy for a while... so many thousands of people are better than me.