I've run across a few issues with the new forum setup, as I'm sure is pretty much inevitable with this kind of transition. I'll collect them here for ease of reference.
*Editing Old Posts*
Though old posts are at least partly associated with my account - it show's the proper post count in my account, for example - I'm not able to edit any of my old postings. I receive an error which says that "The requested comment could not be found." Newly created posts can be edited normally.
*Searching for Posts by User*
In the Advanced section of the Search tab, you can search for comments and discussions by user. Searching for discussions works fine, finding both new and old topics started by the user, but searching for comments only finds posts that were created since the forum transition.
The comment counts displayed in the thread list do not include comments posted to a discussion prior to the transition. For example, the "Premium Accounts Discussion":http://forums.obsidianportal.com/comments.php?DiscussionID=291&page=1#Item_1 thread has 34 comments, but in the "Discussions":http://forums.obsidianportal.com/ list, it's only listed as having one. Note that the comment count listed at the bottom of the actual discussion page is correct, however.
*HTML in Posts*
It would appear that HTML in posts doesn't work at all now. It did, at least in basic form in the previous forum system. This is likely the cause of many broken links, which can at least be converted to textile code, but I don't know of any textile for the _pre_ tag, so lots of code examples will be messed up.
I'll add anything else I find...