I noticed this about installing Mavericks as a download on top of ML. Mail was ( and still is ) incredibly slow to send emails. A simple email, no attachments, can take well over 30" to send. Sending mail to my Gmail account can take well over a minute! The first time I opened Mail in Mavericks there were 136 new junk messages! Never had them before, and they didn't appear on either my iPad or iPhone. Really bizarre! It's like iCloud is not really syncing up all my devices with Mail very well.. It seemed a lot more efficient in ML.
I acted on some of the advice here, like Permission Repair, and starting in Recovery Mode, but in general my system seems OK, except for this Mail issue.
I've noticed a big improvement in cpu distribution in Logic Pro X. That's great. The other odd thing I've encountered is when opening an app for the first time it took a lot longer to launch, but after closing and reopening, the launch - time was much quicker than ML.Apps appear to launch incredibly fast: almost instantaneous in some cases.
All in all, I'm pretty happy ( have a very recent ML back up to fall back on), but there's something odd about Mail, for me. Right now, I don't think I'll wipe and reinstall. Guess it's down to a OS update....
Great thread with a lot of good advice and useful experience-swapping.