In addition to the things that others have mentioned:
Caching
If you page comprises lots of "pieces" - e.g. a Banner pane, Left pane, Main pane, Right pane and Footer pane, and perhaps each of those will have multiple "widgets" in them, then the effort of constructing the page becomes significant (both in database calls, and in rendering at the web server). Having some intelligent caching that is able to detect when any of the underlying content blocks has changed will make a big different to performance
CMS Matrix may be a useful comparison resource of existing CMS products
