A few things to note after I've played with mine for 20 minutes:
1. The "low light continuous drive" issue that is there with the 7D (which the Mk III inherits its meter from) isn't there in the Mk III. It's just as fast in low light as in bright.
2. Live View picture taking is much, much faster. You can actually use it to take pictures in environments with motion.
3. CDAF appears to be improved as well. It certainly feels faster, though I haven't compared it directly to my Mk II.
Thought I'd mention these, as everybody else seems to be focused on other things.
Speaking of focus. Oh. My. God. This thing is heaven. I've never seen AF that can do what this does.
nice on those three points and it does seem that the AF is getting rave after rave