Something not built on WP for starters. At least for me.
1. Ease of use.
2. Speed. No bottle necks. Use of caching, and max. how it loads.
3. Solid, well written, code.
4. FTP uploads. I hate those web admin panels.
5. Customizable. Not all users need the Cadillac of CMS. So it should be modular.
6. Cost effective.
7. Often upgraded, or updated with new features, bug fixes, and so on.
8. Actual support available for customizations.
9. Security. Including blocking of hotlinks, scrapers, auto downloaders, etc..
