I was just about to create a thread about this as well. I have a 50/25 Fios Quantum Connection and YouTube on the Infinity frequently buffers HD video over WiFi. I have tried to diagnose the problem, and have been spending waaaayyy to much time on this. I realize that my current Actiontec modem/router is only Wireless G standard, but still, 20mbps over WiFi should be sufficient to stream HD content.
I connected my laptop via CAT-5 and ran a SpeedTest and got 53 mbps downlink and 33 mbps uplink. I streamed the same YouTube videos in 720p and some in 1080p and had no buffering. Then I disconnected the laptop and did the same thing over WiFi but with different videos to ensure they weren't cached. No buffering at all over WiFI on the laptop. So it appears that YouTube isn't the problem.
What's weird is that I decided to test YouTube on the Infinity while connected to my VZW Galaxy Nexus LTE instead. There was no buffering in the YouTube HQ videos over the LTE connection. So I don't know what the issue is. On one hand, my FIOS connections seems fine via laptop, but on the other hand the Infinity had no problems streaming YouTube HD content over the WiFi Tethered LTE connection. I'm stumped!