Well, some quick first impressions.
1) It definitely lacks pattern noise at higher ISOs. The 50D would sometimes have visible pattern noise (banding) at 3200 and up depending on circumstances. I have yet to see even at hint of this at any ISO. While 3200 is not much "cleaner" than on the 50D it looks better because of this.
2) SPOT AUTOFOCUS! Brilliant move. I cannot tell you how many times I have tried to nail focus on the eye of a small bird only to get the beak or having to resort to manual focus since the focusing sensor is just too big and covers too much. This lets you pick a much smaller focus area so you can really get the focus exactly where you want it with AF. Great stuff.
3) Build appears VERY solid. The XXD cameras are built well, but it is clear that they went the extra mile on this; CF and battery doors close more solidly and so on.
4) Still no mirror lockup button.
Although with Liveview this is becoming increasingly less of a pain.
5) Even faster and more responsive than the 50D which is certainly no slouch.
Will try to do some birding this coming week and see how the AF performance stacks up. Mainly interested in AF point expansion since that seems to be a favorite of many birders.
Did try the 1080p recording and sure, it's neat and the image quality is fantastic, but not sure video stuff is for me.