If you used to play and quit I'm not sure if you'd be into it, because it is still an MMO and it still feels the same. But it's better.
The expansion, Going Rogue, brings a couple things to the table that are cool. First there's a few new zones that encompass Praetoria, the capital city of an alternate (utopian) Earth ruled by a Lawful-Evil (or is he?!?) version of Statesman named Emperor Cole. Dissenters are rounded up by the PPD and disappeared. There's a resistance group, cleverly named the Resistance, which is devoted to thwarting Cole and exposing him for the tyrant that he is; and who, like the PPD and Cole's praetors, are also complete dickholes that usually believe the ends justify the means. So it's interesting, as the storylines play out far from the black-and-white, good-vs.-evil.
And play out they do. Throughout the storyline your character makes decisions that can alter their loyalties from Cole to the Resistance and possibly back again. Besides a couple pure shit find-the-outdoor-glowie missions (Pick Roses for Praetor White? /killself) the new arcs are all quite good, and enhanced by some of the new scripting tech like dialog trees for interacting with NPCs and choices that can affect what happens later in the mission. And there's enough content to keep you busy all the way up to level 20, when you'll choose whether to become a Hero or a Villain.
Cole *is* the bad guy, of course, and as any high level hero knows he has his sights on extending his grasp into Earth Prime. Praetorian characters who remain loyal to Emperor Cole become villains, Resistance fighters become heroes. So no longer are certain archetypes pinned to one side or the other: There can be hero Brutes and villainous Defenders.
It keeps getting better, because the other thing that Going Rogue did, by introducing the alignment system, is provide a way for characters that aren't Praetorian to switch sides, which creates some really good opportunities for getting a richer, more role-playing-ish experience, and adding a lot more mileage for your existing characters, on which I will elaborate.
The game now includes a "Tip" system. Upon defeating an enemy, occasionally you'll get a tip, which is a bit of story fluff that serves as an introduction to an Alignment mission. What is subtly fantastic about these tips is that they give you a choice, adding a bit of why your character is doing what they're doing. For example, you might beat a confession out of a 5th Column solider: Apparently they've got their hands on a dangerously powerful artifact. SO, do you go steal it and gain its power no matter how dangerous it is, or do you go destroy it to make sure no one (especially your enemies) can get it? The first is a more villainous choice (reaffirming your villainousness) and the second moves you towards possibly becoming a Rogue, which is a half-step towards becoming redeemed and being a hero. Once you become a Rogue, you can go into Paragon City and join heroic teams (and do hero task forces), but you're still a villain and can't get hero contacts until you give up villainy for good (well, at least until you switch back).
For heroes that slip toward darkness, the half-step is Vigilante, but the system works the same. The simplest thing, being able to choose the reasoning behind your motives, makes me feel a lot more invested in my character. Even though the system is just a shoe for a mechanical process that allows you to switch sides. It's really cool.
Also they made a lot of other subtle adjustments and additions to improve the game overall. I'm really impressed with Paragon Studios' commitment to making a better game that is not just a WoW clone. (because if that's what you want, just play WoW, right?)