I've heard stories about EVE Online, stories about crazy schemes and wars and how you could do anything, be anything, as long as you invested the time (and sometimes money) in it. This is all very interesting, of course, but trying the game myself was something I needed to do and I understand some of it after a weekend of playing it, but it's not for me, it's quite boring, to be frank, full of seemingly useless systems and confusing progression. Is EVE an MMO? Probably, but not a World of Warcraft style MMO. I'd rather play one of these, they have less freedom in them, but everything you might be able to do is easily understood.
I'm not going to be too hard on Maple Story, its roots date from 2006 or something that old and the game truly evolved through the years. Everything about it got better over time, from how classes started out - at first you had to roll randomly your stats and then fight for 10 levels with a really weak and boring character, while now you just need your core stats and you can skip the tutorial - to the distribution of quests by levels, to the 'balance' of skills to the amount of bugs and random stuff broken here and there. It's not really a good game even tho the concept of MMO platformer is pretty unique but I've spent way much time that I would've with this game during many years.
Guild Wars 2 is not quite an MMO, there is no monthly fee and you don't exactly go around questing for people with exclamation marks over their heads, but it is still a fully-featured, deep and complex online experience where you can explore an insteresting world and feel like there's always something else to do while progressing with crafting, building your character and finding loot.