Ability to have multiple instances above/below each other, next to a single monitor.
-- 1 main monitor in the middle, then 4 other hosts, surrounding it on the top right/bottom right/top left/bottom left, each taking ~50% of the left/right of the main monitor
-- can do this with synergy, if you have a custom config
better, or more reliable, clipboard functionality.
any keyboard/mouse in the system can control any computer.
ability to target certain special keys to a display
-- ex volume keys, have it always target X computer where I have speakers installed, using the audio sharing.
mobile (well, tablet) support
those are just a few that I can think of wanting in the recent past. I'd hoped that a paid product would see some functional updates, as well as some features added, but I haven't seen any in at least a year.