I'll admit straight off that the post I'm about to link to is a bit old -- it's from September. However, it deals with a concept that a shocking number of otherwise knowledgeable WoW players seem to be unaware of: the spawn table. We all know about loot tables, the set of items that any given mob can drop. A spawn table is the set of mobs that can spawn in a given area. Basically, the key concept is this: when the server decides to spawn a mob in some particular area, it picks randomly from the spawn table what kind of mob it's going to be. That means that if an area spawns both Moonstalkers and Foreststriders, like southern Darkshore, and your quest wants you to kill Moonstalkers, it pays to kill every mob you come across.
Why? Well, let's say you only kill Moonstalkers. Every time you kill one, the server decides to spawn another mob. It looks at its spawn table, sees Moonstalkers and Foreststriders, and randomly picks one to spawn. If it's a Moonstalker, eventually you'll find and kill it. Fine. But if it's a Foreststrider that's spawned, you won't kill it, because you're only looking for Moonstalkers. Eventually, the zone will become entirely Foreststriders (unless you're unreasonably lucky), and you'll be scratching your head looking all over for your quest mobs.
Not to mention, you get more XP per hour killing everything you come across (if you're below 60, of course). Anyway, the good people over at Kill Ten Rats lay it out very well, so go forth and be edified.