I recently upgraded one of our testing sites to WP 2.1 Here are some of the issues and fixes that I went through:
1. HeadSpace2 – Needed to replace all instances of get_post_meta to (array)get_post_meta
2. Pagebar – Needed to upgrade to 1.3.1
3. PHPlist Subscriber Form – Appears wpautop has changed in WP 2.1 and is adding line breaks after the form labels. I changed the priority of my PHPList filter so that it executes after the wpautop filter.
4. WP-Highslide – Quicktags were not working because new post are now done in the file ‘new-post.php’ instead of ‘post.php’ as in previous versions. Fixed in latest release 1.22.
5. Ultimate Tag Warrior – There apparently is a bug with this plugin and WP 2.1. Adding a comment and approving it removes any tags from a post. I have not attempted to fix this, but it looks like Mark Jaquith has already found the issue.
On a related note, I was having an issue with the search functionality after installing Ultimate Tag Warrior. This wasn’t due to 2.1, but I’d thought I’d list it here anyway. It appears the join that Tag Warrior adds to the posts search query was excluded any posts that do not have tags from the general search. See more info here.
6. Page Templates: My custom page templates were not appearing in theme editor. After reading this post on wordpress org I fixed the issue by adding spaces to the title parameter in the comments.
7. Categories – wp_list_cats is deprecated, but I decided to continue to use it until it’s completely removed from the core.