In the world of MMORPGs, City of Heroes/City of Villains (CoH) is pretty unique. There are few comic-based games out there, with only the upcoming Champions and DC Universe as serious competitors, and it is uncertain whether those are coming out for the Mac.
Unlike most MMORPGs, CoH does not require grinding, constantly battling enemy characters to gain experience. The primary method for gaining experience is through missions assigned from various contacts. By choosing from the many potential contacts and their offered missions, it is entirely possible to gain multiple levels without once engaging in combat with more than a few enemies. Similarly, you can avoid the less violent tasks and spend your entire time wading through mobs.
The city is divided into two types of zones, City and Hazard zones. The former are relatively safe places with civilians to save, fires, zombie attacks, and alien invasions, but it is easy to engage or avoid a confrontation. The latter, Hazard zones, are desolate areas where villainous groups have pretty much taken over, and the city itself has been twisted - in two cases, completely undermined and collapsed.