WordPress:WPMU Theme Compatibility
Not all themes work well with WPMU. The ones known to have the MOST issues at first try are ones made with Ohz's Theme Toolkit.
In particular, compatibility with the Widgets Plugin is important (but entirely optional) to keep your site manageable as it grows.
The following talks about Widgets Plugin and how to make sure your theme is Widget Compliant: http://mu.wordpress.org/forums/topic.php?id=1541
Themes known to be good:
Theme issues to look for when testing a theme for your MU install:
- hardcoded links
- author profile information
- some issues have come up with the category link (find forum thread), especially if it has been modified by the theme designer
- any extra needed plugins
Themes known to be bad:
Themes that work when hacked:
It is a good practise to test each and every theme you install in MU. Upload it, and enable it just for a test or admin account. Switch to that theme, taking notes of any errors or fixes needed. When you have fixed them, then you may enable the theme site-wide.
Remember, not all themes released have been fully tested or implemented for all situations.