That would be darn helpful-- there have been times in Azeroth where I've killed 20 boars (for example) and then found out there was a quest to kill them that I just hadn't grabbed yet. MW says she worries that a system like this will mean random, indiscriminate killing across the landscape, but I already kill stuff indiscriminately as I'm wandering around-- it'd be nice to get an extra reward for it.
And while we're on the subject, I'd love to see some player-driven quests in the game. Something like 5-10k XP per week that can be awarded by your guildleader, just to keep you busy. Say, "win 3 AVs" or "run Shadow Labyrinth five times." Not only would it let GLs guide what their guildies were up to, but it would be another nice bonus for people leveling up, and considering there's 70 levels to get through now, quicker leveling is always good.
Finally, Mystic Worlds says she's also excited about their XP handouts for exploration, which Blizzard already does to an extent, but it'll be interesting to see how it's done in WAR. On one more (kind of) related note, someone in the guild recently suggested an awesome idea: see how far you could get a character just by finding all the discovery XP in the game. He said level 20, I say no more than 15. Anyone want to find out?