I love explorer areas, I get excited whenever I level a char up to a level that corresponds to an adventure zone.
I think the biggest problem is the lack of replayability of AZs, once you exhaust the "low hanging fruit" slayers. I'm with those who say that you should get the same, flat-rate XP for killing a mob, every time you kill a mob, no incrementation at all...yes, kinda like other games where you get XP for kills and not quests.
I'd like the option for open-ended gameplay, the immediacy of the reward - it means you don't need to invest 15-30 mins even on a shorter quest for a tangible XP benefit. You can spend as little or as long as you like, AFKing if something distracts you, mindlessly slaughtering and with the XP steadily coming in, and never "run out" of XP, even if the XP/minute isn't competitive with the quest options at your level.
Its not like monsters respawn, where you'd have a problem with setting your hireling and pet on hostile and AFKing overnight or anything.