You have to have enemies you love to hate. Doing some street sweeping to knock out that last bit of xp till the next level is great to have, but unless you just want to grind all your levels, we need enemies that people go after with a passion. Mostly I think this would have to be incorporated via story/mission lines. I read through another thread talking about a nemesis for your character and I think that is key, but what about the other factions out there? Maybe my character finds the Street Gang King particularly distasteful simply bases on that enemies disposition. Maybe my character has a holy quest to rid the world of The Order of Magi due to there unholy rituals. Maybe my character wants to beat down as many of the Crime Guardians because they are just to uppity.
A nemesis is someone or group of people that a character fights because they continue to present the most mortal danger to them and their interests. For any number of reasons a nemesis is deadly and can be hated, though will always be respected. I'm not talking about that right now. I'm talking about development of the factions to more than just random mobs in an area, but groups our hero/villains go out of our way to put a hurting on. Give the factions real personality instead of just another check the block, go arrest 100 X type of baddies, go defeat 100 X type goodies.

Just my two cents.

Thanks