Right now I'm playing DDO, simply because it's one of, if not, the best free one right now.
I've also played nearly every mmo there is out there, because I like the idea and hate the implementation in most of them. They're just wrong. So I'm hoping for a true gem to show up sometime... hopefully... if ever.
But I'm looking forward to:
Firefall - Basically a TF2 MMO with emphasis on guilds and pvp. The PVE seems fun as well, so far we have only seen a swarm style of pve where you have to protect a certain machine from aliens. The greatest aspect for me is the horizontal leveling. They compare this to other mmos saying you have a vertical leveling in other mmos, where a level 10 is always stronger than a level 1. On firefall on the other hand, being level 10 will only mean you have more varied weapons, armor and upgrades. Of course, there's a bit of vertical scaling, but they want to keep low levels able to team up and even fight against higher levels on par.
The Secret World - From the makers of Anarchy Online, the MMO I've played the most. It features no levels, no classes. You can team up with anyone, fight anyone, and so on. What happens in this MMO is that you have a deck of 7 passive and 7 active skills and there are over 500 of them to pick from. Instead of levels, you have points, everytime you fill your XP bar you gain a point to put in skills. Also, skills don't level up in the traditional sense. If you picked lets say, Fireball, it will be the same as of someone who picked a fireball after 300 skill points. The only way to make that fireball better is to pick skills in the same group it is. So not only your fireball gets a bit better, but you also get another skill. So you're never just spending points to get one more DPS as it would be the case of diablo/wow-like talent trees, you actually get stuff out of it.