Dingalitous, as some call it, (just us actually, but watch it catch on) is a common affliction where players endure nearly any kind of gameplay grind to reach that beloved level-up graphic and sound. The difference here is that with Call of Duty 4 -- even though players are doing the very same thing they've always done in online shooters -- Infinity Ward added an online persistence and threw an experience bar on the bottom of the screen. Now players watch the little '+5' or '+20' XP gains (for kills, planting the bomb, etc) inch the bar ever so slightly to the right, which keeps them coming back for that next level.
As if COD4 weren't enough, Rainbow Six: Vegas 2 has taken it to a whole new plateau. By giving players a custom character that they can use throughout the game's story and online modes, adding an XP bar on the bottom of the screen that persists in both modes and adding three extra types of experience for specific play styles -- Marksman, CBQ and Assault. So now, not only are players earning that sweet, sweet XP (which gets them new ranks/levels and thus unlocks new stuff) but they're also doing the same for each of these three areas of special experience -- which have their very own levels! That means you get four lines of experience and four different dings, all of which putter along as you play -- at the same time.
So are first person shooters turning into MMORPGs? Pretty much, yeah, FPS developers seem to have figured out that a magical little experience bar on the bottom of your game's screen will cause people to play your game endlessly. Sure there isn't all the usual stuff that goes with an MMO, like crafting or running around a huge world -- but that's all for the better. In RS:V2 you can rappel down the side of a building with your friend, bust trough the window, dispatch a few dozen terrorists and then level up.