Messages goes unresponsive a couple of seconds after launching - any fix?
Well, after a few weeks I finally managed to "fix" it, even though it meant losing my message history on my iMac.

(1) I deleted all the plist and preferences and caches and anything related to messages or chat.

(2) I restarted in Safe mode. I saw that Messages did start up again, as an empty shell, but it started up.

(3) I then restarted in normal mode. 

Messages then started up again. I had to login to my AIM account again, but my buddy list is maintained by them, so that was all there. And I had to reset my iMessage settings. 

But at least it is working again, finally.