Party content appears all over the place in the form of events, which occur randomly throughout zones and 90% of the time will require at least one other person to complete. You don't even have to be in a party to participate, and can chip in a bit then go off and do whatever else later.
There are a small number of instances as well, with level requirements starting at 30. Haven't tried any yet, so can't say much on them.
If neither of those tickle your fancy, then try World vs World, which is a massive collection of around a hundred players in a sort of MOBA format, taking objectives (keeps, towers, supply camps, and even a castle on one area) with siege weaponry like trebuchets or catapults, and showering invaders with arrows or pots of boiling oil.
Besides, it's free to play so if you don't like it now you can pick it up later if you get some friends to play it. I started out mostly solo and now have around ten friends in my guild, so the popularity of the game is spreading a bit.
It is still an MMO, so it's one of those games that if you don't like it, it can't be helped. Me? After playing WoW for about a year (mostly due to my friends keeping in the game), and various other MMOs for very short amounts of time, like the cut of this one's jib. Polished as all hell with constant (almost daily) updates, manages to make all the classes feel very different with different builds between them, a nice crafting system and it doesn't have the exponential time investment to level. Hell, the game released, what, a week ago? And I've already got an 80, but I'm slightly obsessed at the moment.
Also, there's a B12 guild named Kittens, Socks and Magma. Nuff said.