I'm keeping a list as I run across things. Don't expect this to be complete, always accurate or exhaustively detailed.
Here we go: (this is for the dot zero release, btw and keep in mind I haven't played with FCPX very much yet)
1. You cannot currently import FCP6/7 projects into FCPX (see #4)
2. There is no Multiclip/Multicam.
3. Many plug-ins currently do not work.
4. Currently there is no XML Import or Export.
5. Currently there is no native OMF Export or Export to Logic.
6. Doesn't currently work natively with R3D or XDCAM (they need to be re-wrapped first).
An update has already addressed this somewhat.
7. There are no longer distinct Canvas and Viewer windows.
8. You cannot set In and Out points for tape capture nor (AFAIK) is there a batch tape capture ability.
9. You cannot "Save As" to duplicate a project (at least not that easily).
10. Apparently you can't Print (output) to tape, anymore.
11. Currently there is no support for EDL Import or Export.
12. You can't easily open two projects at once and drag media, sequences or custom filters between them.
13. Apparently lacks support for HDMI Input and Output
14. Apparently lacks support for SDI Input and Output
15. Not compatible with many, if any 3rd party video (capture?) cards
16. Timeline seems to always start at 00:00:00
17. Can you keyframe effects? I can't find it.