My vote was for the spawning system, since the lack of one makes the game unplayable and will be unappealing to the mass populace imho.
Personally, I'd be happy with a debugging command to force the issue. If I'm hunting <mob>, I get to the location where I *KNOW* they should be, I'd rather issue a command and have everthing pop at once rather than have to wait 5 minutes on the off chance it might spawn.
My second pick: Targetting system.
It's probably my own fault, but I play a dragon in third person. (I play a dragon. OF COURSE I play in third person.) Problem is, this game's "click the target" method leaves a LOT to be desired for harvesting and especially in combat.
My suggestion as a solution: A key that cycles to "next targettable object" just like EQ2's <which is the TAB key>. When you hit TAB in EQ2, you choose a target, be it a harvest node, or a mob. Hit TAB again, and you get the next object... you can cycle thru everything that's in sight, perhaps spotting things you didn't know could be targetted.
3.) Thinking about it, maybe I need a better explanation of how this works... I've been told (in game) that there's something called a 'spawn object' that, when you get close to it, will cause that area to spawn its mobs. IF this is true, another suggestion would be a debug-style command that makes that object visible. Even if it's just a wire frame, glowing spot on the ground, ANYthing that makes it clear what I have to do in order to avoid having to sit around and wait for the game to catch up.