An incomplete checklist on what should make it to a perfect CMS:

1. Super easy point-and-click installation
2. Intelligent defaults
3. Standards-compliant code generation
4. Pleasant and usable UI
5. Cruft-free URLs
6. Extensibility, plugins that can hook into the API
7. Easy rebranding
8. Flexible layouts and themes, with content not limited to “blocks” or “nodes”. The layout engine must be such that a home-grown CMS or a static solution could be converted to it with minimum of fuss.
9. Intelligent caching, with optional static page includes.
10. Easy content composition.
11. File Management.
12. Theme Management.
13. Pingback/Trackback support.
14. Spam prevention routines, blacklisting, CAPTCHAs and the like.
15. Blogware support, including external posting mechanisms.
16. Easy one-click backup/restore of content on the site.
17. Container styles – CSS rules specific to a given page/node/block of content.