My favorite, actually, is thumbnail previews along the progress bar, so you can see exactly what you're skimming over when you're skipping around a video. The video player itself is 25 percent larger with a new native res of 720x404, so 480p video isn't scaled down (much) anymore, and it's chromeless (the controls disappear).
Other new hotness includes adaptive streaming, so it automatically adjusts a video's bitrate to fit your connection—meaning less stuttering and buffering when your connection suddenly starts sucking—and ad volume normalization, so loud ads won't blow out your ears. There's a lot more, so check out the full photo set of features here. The stuff should be live soon—the original blog post was pulled—but I'm tempted to hammer refresh until it is.