Just a note, even MarioKart may still look acceptable at 30fps; in fact I'd recommend that, as racing games are very high-motion.
I was streaming Sonic & All-Stars Racing Transformed at 60fps and was getting a TON of blocking. I dropped it to 30fps, boom. Blocking fixed, and there were no real smoothness issues.
I'd say that for stuff like Super Meat Boy, and maybe Smash Bros; 60fps could improve the look of the stream. But anything with LOTS of full onscreen movement (not just sliding the playfield one way or another in the same perspective, like platformers tend to) is almost definitely going to look better at 30fps at the same framerate; giving that image fidelity bump as opposed to a smoother-moving, but potentially far blockier/lower-quality image.