I'm a sucker for rank.
Game developers, take note. If you want to get me to play a game and play it for a long time, include unlockable ranks.
Maybe it's because I grew up in a military family and I had military aspirations until said aspirations were dashed at Richmond MEPS thanks to a civvie doctor who was late for his tee time (a rant I'll discuss at a later date), but give me the ability to earn a military style rank and I'll jump on it like stink on shi.. err... poop - despite the fact that they're damned meaningless.
Sure, some people boost for their ranks, some even outright cheat or blur the cheating line to get their ranks, but not me. I want to earn my fictitious rank. I'm proud of my current Lieutenant Grade 2 in Halo 3. But, at the same time, I'm realistic and am not going to spend my life earning game ranks. I don't have the time or inclination to ever go above Captain in Halo 3, so I'll be content when I hit a highest skill of 20 and can work on all of the Captain grades. The way I see it, there's probably not going to be a lot of Grade 4 Captains out there, but I'll be one of them.