Overall, I'm pleased with the upgrade to 3.0 on my MT installations. I am having the same problems across all the installations though -- I'm sure these will be resolved by the next public release:
(1) On blogs where I don't have comment moderation in place - it's still moderating them.
(2) Haven't been able to get typekey to work in my comments - it throws errors about my not having signed up for the feature. Even with a brand new weblog - using their "out of the box" templates - it doesn't work.
(3) When I delete a comment - it hangs on the Deleting Comment window. Never refreshes back to the comment listing page. It does actually delete the comment though - I just don't know it until I manually refresh.
(4) I haven't upgrade all of my templates on all my blogs yet to the new style - - but it really only affects individual and comment templates, so it's not a necessity right now (especially since typekey isn't working anyway).
That's it for now. All of my plugins are still working perfectly and I've had no problems rebuilding. The typekey and comment moderation are the most frustrating -- even with the default templates typekey doesn't work and it still moderates. (Though I love that new features - I do have it turned on for all the blogs on this site).

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