Blizzard is definitely in a rush to get WotLK done. If you read the beta/PTR forums, you'll know that they're triaging changes by pushing off non-critical changes to a post-WotLK patch.
It's also worth pointing out that Blizzard NEVER pounds a stake in the sand like this in terms of public dates.
This is because it was announced to shareholders early this year that WotLK would be coming out in the last quarter of '08, in time for Xmas. The corporate suits have removed from the developers the power to make a good product, just to raise their stock value and cash in on Xmas sales.
Well, as someone who's already moved my addons off the SVN here, I can provide my perspective:
The main reason is that the CurseForge project management is incredibly overcomplicated. I'm not supporting a Linux distro here; I'm releasing a 5kb UI mod for an MMO. I just want a place to enter a title, write a description, upload a picture or two, upload new versions, and get feedback from users. I don't need "pages", I just want a single description. I don't need components, milestones, and multi-author project management. I don't want to have to click through 3 pages just to get to the point where I can fix a typo. I don't want to spend 45 minutes looking for documentation, conducting forum searches, and submitting issue tickets to figure out how to edit my addon's description, as I recently had to do after the basic "edit description" function was removed and folded into the "pages" system.
While the CurseForge ticket system and auto-packaging are nice, the rest of the system is just way more than I need, and as a result is far more difficult and time consuming to use than it needs to be. Since I'm already hosting my addons on CurseForge, and have them all set up and ready for 456723-click version pushes, I don't see any reason to do it all over again on WowAceForge. Also, as I understand, it will only be necessary to host an addon on one site, as it will be syndicated to both.
Yeah, it almost seems like WowInterface is geared to be a home for small addons, CurseForge for large addon projects, and that WowAce the main community thinktank and testing ground.
I wouldn't be too worried about addon sites "stealing" addons. People visit these sites to get updates to their addons, and unethical sites would find themselves not hosting updated versions of anyone's work the next time a major patch rolls around.