I know I will have to re-kill some of the critters in some areas. I've already cleared Mire of the Damned and I imagine I'll have to clear the area around the Castle again by the time I get back there, though I've already done the other quests you mentioned. It's probably going to happen in Kriegspire too, unless I really hurry up. I've got the quest for the superior temple but I was hoping to level up a bit more before I did that. Plus there's the other three dungeons in that zone, and I'm guessing it's going to respawn before I finish all of that.
Also, I'm thinking about going to Dragonsand and killing the couple of wyrms that hang out just north of the little town and then check those occasionally to see when the zone respawns before I tackle it (and Varn), so that I don't have to worry about it re-popping in the midst of either of those. I know that the first time I went in was for the pedestal quest and I think that was around mid-year, but I don't remember exactly.
I guess what I meant is that I wasn't going to set out to clear an entire zone for the extra experience more than once, but yes, I know I won't be able to avoid killing some monsters a second time.
