I have been playing this mission/server type with Wolfie and Bandy and it's a lot of fun after you have gotten used to it. A sector on the map (usually a town I guess) has to be defended. The side that manages to get most people for the longest time inside the circle wins (actually I'm not 100% sure how the mission logic works LOL). When you are inside the circle you earn money/xp(?) that you can use to improve your gear. When you are shot you can be revived (incapacitated soldiers lying on the ground also attract enemies that are waiting for someone to stop by and help them!) or you can respawn. After respawn you need to arrange for transportation to the battlefield by either buying a ride or hop onto someone else's.
This simple game mode works surprisingly well with a small squad. You'll create your own objectives on the way: "let's move around that sector and occupy that building". And of course you have the same intensity as an MP dogfight in RoF or Bos.
I still also want to do the coops though, they provide a more relaxed game mode and you can try out more stuff in ARMA!