Buffiness
This is one of the biggest challenges of DM'ing at higher levels (12th+) is that you must contend with VERY powerful characters who are all made far MORE powerful by the complement of spells they have up during adventures. Even your targeted Dispel Magics will only take down a handful of spells, and AOE dispels can just get one per person. In my experience, the effort as a DM was in taking the time to figure out what "buffing" all the creatures, villians, and NPC's accordingly to compensate and compete with a party of high level adventurers. Monte Cook's high level adventure, Demon God's Fane, is one of the few good high-level adventures that really takes the time to think out all the buffing that would be necessary, and INCLUDES all the buffed up stats. Whew! Look at the differences people talk about to their characters, and these are their lists of ALL DAY buffs, I'm sure it gets much worse (as it does for my players) when you throw in the list of "big combat" buffs like Divine Power, Righteous Might, Divine Favor, Holy Aura, etc. that only last 1 or 2 rds/level. Whew!