The first thing that jumped out at me as a new smplayer user was the large icons and toolbar which took up alot of valuable vertical space.
I found that changing the control bar to mini gui helped but the menubar and a few other things
1. Option to remove menu bar - the menu bar is superfluous most of the time since the right click also brings up the menu.
I saw a few similar requests in the sourceforge discussion.
2. The large icon size (minimum 24) takes up more space than is necessary, and there is excess space above/below/btwn icons.
3. Having a way to put the time display (time label action) just above the time slider bar (without increasing the vertical height) would help save space by not shrinking the slider bar.