I’ve been trying to pinpoint exactly what it is that I love(d?) about Rift, and I think I figured it out. It was perfect for an introvert like me who enjoys solo play, but also wants to do group activities without a lot of hassle. Rift’s dynamic rifts and zone events fulfilled that requirement perfectly.
You’re out doing quests to level your character, enjoying the scenery. Then a zone event breaks out, and you rush to the spot marked on your map and find a big group of players there. Ten seconds ago you were by yourself in the middle of nowhere, then in a flash you’re part of a group working together toward the same objectives. You can heal if you want to, you can tank if you want to, or you can just sit back and pew-pew if you want to. You don’t have to spam a chat channel with "lfg" for an hour, you don’t have to sit in a dungeon finder queue, you don’t have to manage a bunch of personalities, you just run to a spot and you’re automatically part of a like-minded group working toward the same goal, without any sense of commitment. When you’re done, you just go back to your quest. It worked beautifully. (You could even ignore the event completely if you wanted to be left alone.)
GW2 has a very similar mechanic with their dynamic events, and I loved doing those while leveling (being able to resurrect fallen allies is awesome). FFXIV has FATEs, and I enjoy them too. Any game that has a way to work together toward a goal without having to "commit" to a formalized social structure is going to appeal to me.
In the next evolution of this concept, they need to find a way to automatically add nearby players to your raid frame so you can heal and buff them without the need to even form a public group.
Undoubtedly all of that makes me sound totally anti-social, but I think it’s more of an introversion trait. (Introverts often look like anti-social jerks to outsiders.) It’s not that I dislike formalized groups and guilds–I did a bunch of guild raids in Rift–it’s just that there is a much higher level of commitment in that, both in time and energy, which I find draining.