Since GM 1, my Messages app wouldn't work properly and for the life of me I couldn't figure out why. It would start up but no windows would show up. Couldn't see the main setup window or conversation window either. Only window that would appear was a separate "New conversation" window if I chose "new message", but everything else was broken. In the Guest account however, Messages worked fine. Then I noticed in Console an error regarding fonts when launching Messages from my user account. Anyways, so I had disabled a bunch of fonts in Font Book leaving only the essentials that I wanted. Apparently, Messages needs some non-system essential fonts enabled (helvetic, lucida, emoji, apple gothic and 1 or 2 others) or else it goes wonky. In my case, I had "Apple Gothic" disabled and that's what caused Messages to stop working. Just sharing in case anyone else is having trouble with Messages in Mavericks and has some fonts disabled.