There are some absolutely amazing HTML5 / WebGL things out there; I'm just not sure you've seen them. It's true that browser incompatibility is an issue, but say you're ONLY targeting Chrome desktops, which still has a bigger user base than most consoles--if you're doing it right you're getting hardware accelerated graphics and the performance is pretty great. Have you seen cloudparty.com? It has multiplayer in-world building, skeletal animation, particles, full screen effects... the works, and the performance is definitely up to snuff. Observe tons of realtime screencasts here: http://www.youtube.com/user/CloudPartyInc

I think that the problem is that WebGL is young and there are LOTS of ways to shoot yourself in the foot perf-wise. Only a few teams have really figured it out and the rest are making the tech look bad. But the performance is there if you're an experience graphics programmer and you know your way around JS wackiness.