Close button sends client to the tray, statically linked Qt for OSX.
See the Changelog for list of features and fixes currently only available in the nightly builds.
I agree, and I'd add some ideas.
There should definitely be options for different cases. For example I'm glad when someone download my favorite album, I'd shutdown when it be finished BUT, it should be middle state between 'shutdown after downloads finished' and 'shutdown after uploads finished'
Shutdown on current folder|user download finished etc.