Many people avoid other games in the genre because of their steep entry level. Unsurprisingly, Blizzard has gone to great lengths to lower that bar to help players get in while balancing certain aspects to raise the skill ceiling.
Team experience and leveling reduces blame and griefing found so basically in similar games, meanwhile talent trees feel less daunting to manage than a complex economy path. Add onto all that Blizzard's familiar character set, and you have a great game.
