Kill as many as possible in X amount of time.
There are a number of quests that feature objectives that are to defend again the hordes spawning at a steady rate. That is all well and good, but as others have noted, for a well equipped group those objectives can end up being tedious as they end up waiting around between spawns.
So what I was thinking was, maybe it would be interesting to have the quest mech tweaked a bit. Maybe instead of Kobold Assault being, kill 200 Kobolds, maybe it is survive the Kobold waves for 15 minutes. And Kobolds just keep spawning as you kill them. At that point an option objective becomes "kill 10 Kobolds," then "kill 20 Kobolds" etc. So the more you kill the more XP you get.
I use Kobold Assault as an example (as I ran it last night and, as always, killing the last 20 is tedium incarnate as the spawn rates drops dramatically.) I am not suggesting retro fitting, just an option for future development.