I went back playing some DC Universe Online recently.
I was a Legendary member for some time, it expired since, and I'm back to Premium (there's three account types, namely "Free", "Premium" and finally "Legendary", which includes everything). I still enjoy the game overall. I got some PvE characters and two PvP ones (both Level 30, which is the maximum level right now). I have joined just one guild so far with one of my characters (Villain in the PvP server) but I haven't really done anything beyond the main PvE quests. I can't comment on the "end-game" much since I've only completed one or two "dungeons" so far and I only play occasionally. But, overall, it's a fun game, I really like the combat system and the "Tap / Hold" mouse key system to come up with combos and consecutive hits in specific orders. It's awkward at first though, you don't have a mouse cursor on-screen and you don't manually click on other players to see their levels and stats or to inspect them. Instead you have to open up another menu (one of a few menus via the 'F' keys, F1, F2, etc) and select specific options from a drop down menu. I was annoyed by that during the first day or two when I was new to the game to be honest, but over time I got used to it. The latest version includes basic self-owned bases for your characters, it's a neat feature and they actually serve game-play purposes outside of role-play and aesthetics.
If you don't like super hero themes and comics of that vein (similar to but not exclusive to the DC universe) then you might want to play something else though. It does play like an MMO, but definitely the themes of "hero Vs. villains", along with the general presentation (including graphics), voice-acting and the types of missions you have to do (of course mostly related to events that occurred in DC issues from what I read around) will always remind you that you ain't playing something "else", it certainly does a good job at keeping you immersed in the DC universe that's for sure. The one thing I have to say that stands out in the game, probably above most of the features and game-play mechanics is, ultimately, the voice acting. The cast of voice actors and actresses is impressive, and many of them have done a superb job for their respective characters. From Adam Baldwin (Superman), Corey Burton (Brainiac, the "main" antagonist in the game), Kevin Conroy (Batman), and Michelle Forbes (Circe) along with the famous and inevitable Mark Hamill who, in my opinion, is most likely the best voice actor in the entire game voicing The Joker (seriously, absolutely amazing voice acting). I am not exaggerating when I say they hired 25+ talented voice actors and actresses. Many MMOs out there are often lacking in voice acting variety, if not talented voice acting, but DCUO is far, very far from being one of those.
Additionally, regarding the voice-acting (indirectly) is that if you're a Hero or a Villain you'll then choose which one of the three "mentors" will guide ("order") you around to do various missions. As a Hero you can go with Superman, Wonder Woman or Batman (Batman is awesome, do I need to say that again?). As a Villain it's business with Lex Luthor, Circe (never heard of her before playing the game itself, although never was a "fan" of the DC universe prior to that either, but still never heard of her prior to the game, she's decent though) or the Joker (seriously everyone playing DCUO needs to make a Villain just for the fun playing along with the Joker and his hilariously vile missions). Another thing I really like about missions as a whole is that each one of the total six mentors have their own respective main story arc and they all deal against one another within that arc. For example, Superman is after Lex Luthor (which is obvious), Circe really, really doesn't like Wonder Woman, and Joker keeps trolling Batman (business as usual). And you're finding yourself between their personal battles doing their deeds as their newly found precious "champion" (Circe can't stop calling you her champion, I suspect she has a crush on you by the end lol).
Anyway, I actually really like the PvE and questing in general, most of the story lines follow at a nice pace and most of the time the chronological order allows you to quest in both Metropolis and Gotham City. And the one last thing that perhaps needs to be mentioned about DCUO, which I really like, is the fact that there isn't like 20 freakin' zones to separately load which in turn would split the population in small groups, which in turn would provoke the impression that the game's population is low. For the PC you basically have two servers and that's it, it's not like GW2 where the players base is split between like 20 servers, and then each of them further split the players base between all those seemingly unending zones. It's either PvE server, or PvP server, that's it, but the game's population is constantly playing in either Metropolis or Gotham City which are the only two separated zones of the game. You have other various smaller "hubs" (bases if you will, such as "night clubs" or police stations, etc), and one main base for heroes and villains. But other than those hubs the actual "game's zones" are Metropolis and Gotham City, which themselves are completely seamless. You can literally fly, run or jump and climb (acrobatics movement mode) from one end of the "map" (say Metropolis) to the other without loading anything whatsoever. The map's objects and various visuals just load at generously large-enough distance away from you as you move around (only visible if you fly high enough to see everything and the city skyline and so forth, with some buildings fading into view at a distance).
Anyway, there's other features I like, it's not exactly a "mind-blowing" game, there are some things I don't like that much but generally speaking it's a polished MMO. There was a time during which it was subscription-based, which explains why it's perhaps a bit more polished than your usual completely F2P MMO around. They had the budget to at least make sure that from the start features and mechanics were fun and polished enough. By the way, yes it's completely free to play, but if you only spend $5 in the cash station (marketplace) your account will permanently be changed to a "Premium" status (which has more character slots, more inventory slots, and a few others things), and you'll never be reverted to "Free". The only membership you can choose is Legendary which includes everything and the DLCs so far released. The only "real" inconvenience to not being a Legendary member, in my opinion, is the in-game currency limitation (which is $2,000, which is absolutely nothing really, you'll probably have that much by Level 10 via regular questing and selling junk as you progress), so if you want really good items via trading you pretty much have to trade other items in exchange rather than in-game currency.
But that's probably just a problem once you get to end-game, and if you join a guild then perhaps the problem is further alleviated from the help of fellow members, but I can't comment much on that. Anyway, the thing is it's free to start with, you could always just download it and give it a try for what that's worth. If you (or anyone else reading this) aren't in the whole hero Vs. villain stuff that's one thing, but the game itself is an MMORPG and that's what you're looking for. It provides good and polished enough features, plays smoothly and is just fun, generally-speaking. The only way to make sure about whether or not you'd like it or end up bored after doing one or two missions is to download it and play it. As a side note I personally believe that playing as a Villain, in your case, might be a better starting point, simply for the "refreshing" atmosphere of not playing as "yet another hero" ready to save the life of that next meaningless pedestrian in danger while there's other pressing matters around. Go with the Joker as your mentor and start playing in Gotham City, which has a nice gritty and darker atmosphere (and the soundtrack fits that mood perfectly). And have fun doing the Joker quests with a smile on your face and the best cynical weaponry against the forces of "good" you can think of.
Anyway, I was in your boat as well recently (looking for new'ish MMOs around). But most of the ones I try are boring. The only ones I "tolerate" (at worst) or find "alright" (at best, mostly) and play occasionally are GW2, DCUO, and Champions: Online. I started playing FireFall recently, it's decent enough so I'm keeping it installed, but it's still under development (beta right now). I'm also going to give TERA a try this week since it's going F2P.