I would say it's game mode, game mode, game mode. I love the game, a lot. And I'm getting quite bored of Solaris Arena. Once there are other game modes, I highly doubt I'll ever go back to it. It's just too simple and doesn't offer any sort of rewarding goals IMO. Bought BFBC2 yesterday though, because I can't keep shooting it out with the same people without goals endlessly.
I think population will pick up more from game modes than any other option/feature, including the mechlab.