From my experience working with Steam's broadcasting platform, Twitch, and Youtube... I'm not sure if Youtube has the ability to really compete directly with Twitch. The easiest comparison was during the Dota 2 International recently. The tournament was broadcast on all three at same time.

Twitch was pretty much standard experience. It didn't crash or lag out more than the other two, nor was it stable when the others were down.

Youtube had a nice interface, it was about the same visual quality, however...it was 20-30 seconds behind, due to the HTML5 encoders. This is the primary reason Twitch hasn't made a full transition to HTML5, as they've been trying actively trying to reduce stream delay as much as possible to promote chat interaction. However, in Youtube's favor, it did handle the 60fps aspect of the broadcast swimmingly.

Steam's platform really, really impressed me. To the point, I wish I could go to Valve's headquarters and ask them what the hell they did differently. The stream delay was exactly the same as Twitch, however the video quality was noticeably better. Frame rate was as steady as Youtube's while the contrast and overall bitrate were much better. Plus the platform has the ability to rewind live without having to skip anything. Once you're done with your personal instant replay, hit the "Live" button and it instantly syncs you back up to the broadcast. Youtube claims to offer the same, but you know, no hands on experience on that one.

So... while people keep making noise that Youtube's the Twitch competitor, I really feel Steam's going to quickly take over as primary challenger. As soon as the personal broadcasting via the Steam overlay gets stable and out of beta, it's going to actually be easier to stream on than Twitch.

Going to be fun to see how all three companies start leapfrogging features :)