Good things to do:

Better enemy AI
Increased power for attacks that can be avoided by simply moving away (example: Raptor Breaths)
More enemies with different kind of moves (debuffers, healers, etc).
Increase difficulty in non obnoxious ways (IE, doubling enemy HP and calling it a day)

Bad things to do:

Forced parties to do any XP progress at all (IE: FFXI). This doesn't mean parties shouldn't make more XP than solo, this just means that a solo player shouldn't have to level up several times slower than a party.

Deleveling on death. IMO, they should increase the current durability loss when you reach higher levels to a point where 2-3 deaths would be enough to make an item go from 100% to 0.

Long cooldown timers on content, other than few, very specific battles (and counter that with higher drop chances.)