Mail not fetching new messages
Mail remains flaky in DP3 - after launching, it'll fetch mail in my various accounts for anything from minutes to hours. No errors are reported - mail will simply cease arriving in any or all of the accounts. The workaround is quit & relaunch - but a mail client that doesn't actually manage to retrieve mail seems to have a bit of a problem.
Curiously, Mail was absolutely fine for me on that front up to about 10.9.1, where this began happening. Yosemite DP1 was much worse, failing to fetch further mail all but immediately; DP2 improved things, but still, if I want to be sure it's all working, I have to quit and relaunch, and watch all the "new" email arrive suddenly.