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.
Whew, that took a lot longer than I expected. To wit!
Download alerts: You can now instruct SoulseekQt to alert you when the download of a file, all files in a folder, or all files from a user are finished downloading. I expect this will need some fine-tuning to account for file types that may not be processed by the uploader, thereby making downloading a complete file set possibly unattainable. Until then, right click a download and use the options provided under the 'Alert me when...' menu to set the right type of alert. This will flash the application icon when the alert triggers as with other types of events, but also pop-up a bubble notification under Windows and (at least the Debian flavor of) Linux. Qt doesn't appear to be able to do that under OSX...
Chat logging: All room and user chat windows now feature a 'Chat Log' button in the action bar which will pop the log in a searchable tab. Search is only very basic, type what you're looking for and use the arrow buttons to navigate to other instances of found text. An 'Open Log Folder' button will take you straight to where the log file is in case you want to back it up or move it somewhere else. A 'Clear Log' button will delete the log file (but keep logging chat in the room or with the user). If you want to disable logging altogether, you may do so under the Options->General tab.
Color configuration: This appears to be very shaky under both OSX and Linux unfortunately. Under Windows it seems to work perfectly well. Use the 'Select Application Colors' button under Options->General to open the Color configuration dialog. Colors may be changed for the various basic UI components as defined by Qt (Window/Input/Buttons), as well as transfer statuses and chat elements.
As is often the case, much else has changed under the hood, so new bugs not in the vicinity of this release's major new features are possible. Links on the download page!