Well for me, I like the genre. The typical fantasy MMO is overdone. Granted the classes are similar (blaster = mage, stalker = rogue, etc) but it's the setting here that I enjoy. I want my character to be a superhero.
But it's more than that. One of the most important things for me in an MMO is character progression. I get a new power and my character feels more powerful. More slots and my character's effectiveness is increased and it's noticeable (provided I can put an enhancement there). This happens through out your leveling experience.
Also, another great thing about this game is the powersets play differently for the most part. Yeah some are similar but instead of defense the set affects damage resist for example.
And lastly it's simple. The game isn't overly complicated in what it is and what it does and how you do it.