I predict that extensions.gnome.org won't be quite as useful as hoped. Many extensions will suffer bitrot, updating will be tedious, and it will acquire a reputation for instability. Also, it would be interesting if a security problem or two cropped up.
An effort to produce a nicely-integrated set of extensions could succeed, especially if curated by a popular distribution. In fact, that would be excellent.
But, I'm not sure who's going to do that... With Cinnamon and Unity forking clean away, it doesn't appear that many downstream developers want to tie their projects closely with Gnome3. Understandable.
On the one hand, it's a real shame that the Linux desktop has been thrust back into such deep uncertainty. On the other hand, 2012 will be a fun year to watch!