Guild Wars 2

Right now, grouping in GW2 makes this game leaps and bounds ahead of other MMOs. LFG tool is something that you can use for almost every PVE and PvP aspect. And it works beautifully well. You can use for any simple thing you could imagine and making a group call or joining in, is as easy as it can be.
There are some group announcements that demand certain level of AP, zerk gear, experience, beautiful girls or cookies. But very little calls for groups with only certain classes or gear. So, anyone can join and rarely you get into a group where you get kicked out due to your class, gear or traits.
This makes causal and pug playing a joy and even helps to bring guild or pre-made players closer to others.
SW is an excellent example of how great grouping is in GW2. People are constantly making group calls for events of cf and literally anyone can join. It is a breath of fresh air in the MMO world which is mostly plagued with a problem of not enough tanks and healers and too many DDs.

Is this what we should expect? Calls for certain classes and/or gear becoming mandatory in grouping?

For raids, probably, not likely for much else though.