nolanfc wrote:It's popular because the experience-and-unlocking carrot dangling in the face of Joe Average keeps him engaged until he's played long enough to improve his skills and start enjoying himself.
It's worth noting that while the above is certainly true, it obviously isn't enough merely that this leveling system was present, but rather that it was extremely well executed by not unbalancing matches where there were large level disparities between players. This is because, for the most part, the early level weapons and equipment are the best all-around stuff in the game. The best all-around assault rifle, for example, is the M16A4, which is the very first one you have access to.
The brilliance of the system is that when you level up, you don't unlock better equipment, you unlock more specialized equipment. Later assault rifles do more damage, have longer range, or are more accurate, but never all at the same time. You can specialize more thoroughly in the role you want to play for your team, but at level 55 you can't build a better general purpose rifleman than any low level player can. You can build a better sniper, or a better stealthy smg-user, etc., but even that advantage will only be useful if your teammates are coordinating their roles with yours.
nolanfc didn't meantion the killstreak rewards, either, which grant a player the ability to call in a 30-second UAV (all enemies visible on the minimap), an airstrike (player designates target on the map), and an autonomous attack helicopter that patrols the battlefield for 30 seconds, at 3, 5, and 7 kills without dying, respectively.
The interaction between the killstreaks, the minimap mechanics, and your equipment selection is important, as well. Initially, only friendly players show up on your map, as green (or blue?) dots. Enemies appear for a few seconds as red dots whenever they fire a weapon. Melee attacks don't reveal you, nor does firing a weapon with a suppressor (but a suppressor replaces any kind of aiming enhancement, like a red dot sight or scope, so you can only aim down the iron sights and with less zoom). When a UAV is called, all enemies appear (unless they have selected a perk that makes them invisible to UAVs, at the loss of a perk that might enhance their health or damage). This ties into calling airstrikes via a full sized map that temporarily overlays your screen, and obeys the same rules of revealing enemies as the minimap; if a UAV is active you will obviously have a much easier time choosing a target that maximizes the effect of your airstrike.
At any rate, I'm just trying to make the point that the multiplayer successfully intregrates a number of different mechanics and has a not-insignificant amount of depth, especially for a console game. The fact that the depth is more a consequence of the equipment selection and teamwork possibilities than a player's physical reflexes and precision, which is necessarily limited on a console by the nature of the controls, was an excellent design choice.