Better access control level (ACL) system
The ACL solution is a no-brainer in my opinion. Until now there have been many 3rd-party components developed to solve these issues, but they all seem to fall down in one way or another. I’m very curious as to how the new ACL system might work with LDAP, and if it will be possible to sync joomla groups against OUs. Joomla is a legitimate contender for the intranet space, this will be critical.
Nested categories
Combining the above ACL system with a nested category structure will increase the power 10x on projects where you might want to mirror your company org-chart including teams and virtual teams. You get the idea.
Standardizing extensions against the MVC framework.
I really feel like this will make life much, much easier for 3rd-party developers to develop cool add-ons for joomla, given that several MVC frameworks have really jumped to the fore and done a good job separating the presentation and data layer.