I started playing EVE online on the winter holidays. It's a world of itself, and quite intentionally so. Thousands (actualy 5431) of star systems, each with planets, moons, asteroid fields and so on... with starbases, planetary colonies, moon mines. Flying in the space in between are traders and pirates, smugglers and cops, mercenaries and businessmen, researchers and explorers. No classes, no set path to follow - you pick your destiny, it's a sandbox game.
And it is the first game where the "massive" in MMO was true. Whenever I logged in, I saw no less than 24000 players, most of the time over 40000. They are in the same "shard"; it's not divided in separated instances as most other multiplayer games are.
Most of the game is player created. You can go solo, mining away for riches, trading on the market of far worlds or scouting for hidden riches - but most people group with others in corporations, which are run like businesses. And those businesses form alliances. And almost inevitably, those alliances go to war every now and then, over the control of star systems, groups, even constellations, with all that entails - rallying, logistics, diplomacy, spies, counterespionage, sabotage, and of course space battles. It's all there - and you can pick what you like.
For my personal taste, I started "mostly harmless" - a bit of mining, a bit of trading, some production. Of course I'm armed and learn some fighting to avoid getting robbed, but it's predominantly escaping skills so far. And there are many skills to be learned for my character...
... and lots, lots, LOTS of other things are to be learned by me, the player. I may need only a small part of it, but I'm curious, and my curiosity gets satisfied. EVE online has been around for well over a decade, been improved all the time, and it shows in many places.
The User Interface, however, has its pitfalls.
In case you want to try EVE, there's a trial key players can give out, and I have one important *warning* for you:
During character creations, there is a lot you can do to make your char look like you want. Many details of this you can change afterwards - but there is one thing you can NEVER change in this game, and that's your characters name. So be really, really careful about this, and check everything twice, including spelling and all.