The only plugin I’m having trouble with in WordPress 2.1 is the FlickrRSS plugin. Ultimate Tag Warrior also had problems with WordPress 2.1, although I didn’t experience any of them. Ultimate Tag Warrior wasn’t slightly broken, it was trashed, and would delete tags for posts. Here’s what the guys at NeoSmart had to say:
Ultimate Tag Warrior is completely broken with WordPress 2.1 and you can lose all your tags by upgrading to WordPress 2.1. In the more recent SVN revisions of WordPress 2.1, any time a comment is added, deleted, or unapproved, you lose all the tags for that particular post.
Thankfully, Christine, the Ultimate Tag Warrior author, has released a new version of UTW that’s compatible with WordPress 2.1 (I think). I believe the new release implements a fix that was published for the UTW/WP2.1 issue back in December. There still seems to be some speculation over weather or not version 3.1415926 of UTW fixes all the problems with WordPress 2.1. I never had any problems with UTW and WordPress 2.1, even with versions prior to 3.1415926. Sounds like the problem may be somehow related to Akismet from the discussion going on at the UTW forum.