The length of battles, the pace of combat... and the overall feel of combat.

[Morningstar]

Elite monsters. Higher difficulty and substantially higher reward to justify the effort for parties.

There you go, I just found a solution in 20 seconds. I imagine if the devs actively think about it they could do a lot better.

I agree. We need "party mobs" that are specifically meant for groups of players to take on. These mobs have higher hp and stats than normal mobs their level and return higher exp. These mobs will be located in strongholds, dungeons, and any other designated group exp spot.