Nifty GUI lacks a "toggle button". A button that can stay down, in toggled state. For example a game that has game speed selection; pause, play, double speed... The buttons could be toggled. So that the player can see what speed has been set.
This grouping of toggle buttons could be handled in user code as well. But more sophisticated solution would be that Nifty takes care of them. Using properties like group="" and listeners like onToggledOn, onTogledOff... Well, I'm sure you'll get the idea :)