Smoothing the power curve, prolonging the sweet spot, high level play/high level prep.
The Math stuff-- not the fluffy, creative, wouldn't-it-be-cool-if stuff.
Here's a good list of stuff ENworlder's collected as their big gripes. (Not all of this qualifies as hard stuff.)
http://www.enworld.org/showthread.php?t=222801&highlight=fixing
