1. This is in BOTH WP15 and WP20. If you have categories (let's say 25) and you delete every category but the default one, the default one will add all the previous posts together into the main one.
Here's what I mean. I have 200 some posts. I went through and deleted 24/25 categories (except default) and when I was done wordpress was reporting that the category had 690 something posts listed. There were only 200 posts so the actual posts werent copied but wordpress was reporting the wrong number. It took the number it new that the category held and when it moved each post to the default category it took the current number and added the number that the deleted category held and combined it with the default category.
Hope that was clear.
2. After I moved all posts to the default category I can't delete any posts. It gave me some big long error (via JS) saying that my SQL syntax was messed up. I am no longer running 2.0 so I cannot copy the errors. Sorry.
3. Either PHPExec has a problem or WP2 has a problem. I cannot execute PHP inside a post with PHPExec. I have defaulted in making a page with a custom template where I execute the php (the archives).
RC3 seemed very buggy to me (not to mention that upgrading to it was a royal pain).
4. If you do a fresh install and then import:
The comments go bezerk. It was giving me errors in the WP-Admin so I ran upgrade.php but after that I had no comments. Everything was 0. Unfortunately.
These are the bugs I have found. If I am supossed to report them somewhere else please forward me there.