Unit Experience would be a damn good addition.
Instead of just aimlessly throwing units at the enemy like cannon fodder, and winning by sheer weight of numbers is just too crap. It takes away from the "Strategy" in Real Time Strategy.
With units gaining experience, they would be better at their job, and be more valuable to a player. That adds to the strategic element of the game.
Probably why it was introduced to RTS games in the first place.