I like this idea but I think success may be limited to games that are highly visual in nature. In fact, the same may be said of startups in general: video is better than images and images are better than text. BTW, the Google talk by Luis on these sorts of games is a must-see: http://video.google.com/videoplay?docid=-8246463980976635143
I'm curious, why do you think that visual games are more likely to be successful? His question answering games seemed useful and engaging, too.
There must be great startup ideas lurking behind this idea of games and Human Computation. (Luis alludes to as much, with his tongue-in-cheek suggestion that the ESP game double as a dating service.)
