I don’t generally just toss up a link and walk away, but Nacin nailed it in one, for what you need to watch out for with WordPress 3.3. I’ve been SVNing it for the last couple months (very happily I might add) and only had one issue with a plugin. Use Google Libraries stopped working for me.
If you happen to be testing WordPress 3.3, or want to, the second Release Candidate came out this morning, and I’m keeping a Master List of all known issues, to be posted in the forums as soon as 3.3 is really real. Any help finding bugs is appreciated!