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

[Sephr]

I can understand it to a degree, but right now we are facing a many vs many environment. Since mages take sleep much high in the level mobs need to be disposed quick.
A possible way to fix it is lowering level of sleeps (like level 15) and double exp gained, so exp don't become slower.
That's another thing to understand btw: this is a game where you can solo your levels. How you can solo a mob so strong you need 8 people and 5 minute to kill it ?

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.