I'm wondering also. I switched from cable to Fios 50/25 recently. I never had a problem with loading youtube videos with cable, but as soon as I switched it has been hard for me to even get videos at the lowest quality. I thought maybe it was just coincidence and the youtube servers became slow at the same time I switched, BUT if I have my mobile phone connected through my Fios wifi I can't get the videos to play very well, but the second I turn off wifi and the 4G kicks in the video plays fine.
I've done multiple speed tests with positive results and other video sites work fine at the best HD quality.
It's YouTube. If you know how to switch the cache server you're hitting on YouTube's end it'll move along just fine. If throttles were in effect you'd find even that trick wouldn't work. Verizon and Google do need to work together to resolve this issue however, since it's a pretty common issue.