Mavericks - USB to HDMI Monitors
Nothing that I have run into personally but I just ran across this:

http://arstechnica.com/information-technology/2013/10/os-x-mavericks-breaks-multi-monitor-setups-with-some-usb-displays/

Which indicates major issues witht he author's USB monitor (DisplayLink) so I would not upgrade untill you are either sure you have updated drivers or can do without your second monitor.