Twitch has a pretty good player that they've kept around for a while. They've got services specifically tuned to do what they do - streams. The adds line up (mostly) with where it's natural to have adds, the live chat works well, and the streamers have a lot of power over how to run their channel. Most importantly, Twitch seems to not fix what's not broken.
Youtube, on the other hand, has managed to make every single part of their site worse at one point during it's history. Comments are shit, video playback is buggy, prebuffering has been partially removed, I stopped getting interesting suggestions for non-subscribed content, selecting a new resolution takes ages to take effect, resizing the video to small resets the resolution to shit, etc. They're just plain unable to let something good stay good. If youtube people takes over twitch, we will probably see the same bullshit there.
