When it comes to RPG's I hate playing in other established universes and timelines. So, IF I'd ever do a MMORPG..... I'd prefer Champions.
DC Universe... it looks aw-ight. And I think the property is more likely to draw more subscribers than Champions. Not my cup of tea, but if they do it right, PS3 could have quite the powerhouse on their hands.
You can't be those characters, they already exist. You'll get to meet them as the game progresses, like helping them fight their adversaries, or helping their adversaries fight them, or just wreck the **** out of metropolis and Superman will show up.
You play as your own hero, you have a basic power set (movement [flight, acrobatics, speed, teleportation], abilities etc), and you level up to improve those abilities and learn new moves. The 'story' is about why so many new metahumans are showing up.