I've played both Frontlines and Modern Warfare on my 360 this past week, and the differences between them are staggering. Each game has advantages over the other. Many of the differences are of style, rather than quality.
One key difference is serving hosting. In Modern Warfare, players host the games, so games are often interrupted or ended before they even really begin. It's very annoying, but that's not what I want to talk about. What quickly becomes clear is that hosts often end a game short because they do not like a particular map. In any competitive multiplayer game that moves automatically from one map to another, players complain about maps. But that people cut games short, knowing they'll get an earful from those their playing with, shows just how much a map can be disliked. That begs the question:
Is it not possible to give players complete freedom in which maps they play?
Of course, a player can simply leave a current host when a bad map comes up, but he'll have to leave again with whatever host he finds next. It's annoying. That would also mean abandoning the fellow players who he has hopefully been building a rapport or rivalry with (a skilled enemy player can be similar to a "hero" or "boss" enemy in single-player gameplay). The ideal would be to enable a player to have an unbroken play-session without any content that player dislikes.
With a game as popular as Modern Warfare, the solution seems simple. Allow hosts to customize a map series by simply selecting checkboxes, then allow other players to see that customized list before joining. With eight or ten maps, there are many possible combinations... and that significantly divides the player population. But with a player population of over a hundred thousand at any given time and fewer than twenty players per map, I doubt it's a problem even with matchmaking dividing players further. That's something Infinity Ward should consider for MW2.
But most games don't have such massive player populations. Even MW will get old and its populations will dwindle. So what's a solution for them?