Personally I find grouping a poor substitute for in-game social interaction. That sounds mad but let me explain.

I'm sure we've all had the situation where you're in the process of killing X of Y orcs for a quest and another player arrives and begins doing the same. They ask if they can group with you, do so and then the pair of you kill - pretty much in silence, as you're busy - before the other player declares "done c ya" and quits the group.

That's not a social interaction and I don't think questing should be the core of a game's player-to-player interaction.

Rather, I prefer to see more hub-based activity, where players can chat without worrying about an orc killing them or if they're stabbing the right buttons. As I've mentioned before SWG did this brilliantly with the cantinas/med centres.

Or there's the example of player dependency, where Player A can make an item but to do so, he needs an item Player B can craft. This sort of cross-polination of crafting (especially when crafting is important and customisable) makes for excellent levels of player interaction. Again, SWG did this well. I'd often find people asking for specific items or engaging in conversation about items I could manufacture, placing bespoke orders and so on.

This is where social interaction TRULY flourishes. Not on the battlefield but away from the action, where players are not forced to group to overcome an obstacle but rather they interact willingly to reach their own personal goals.