Some people seem to misunderstand that not ALL games are trying to go for the "team-based combat", or "tactical" appeal. Call of Duty is about getting lots of kills, and that's about it. It's like building up your Whatever Meter in a fighting game. It's a simple system, which is why it doesn't need any of the complex versions people are suggesting. Crysis 2's dogtag system is interesting, but finnicky in that it involves finding a tiny object in the middle of intense combat; in some ways, distracting from what you want the game to be about.
I wouldn't expect other games to go for the killstreak system as much as they do; in non-deathmatch-themed games, they don't fit. And even in Call of Duty, they've struck a balance where the rewards are usually somewhat defeatable. (And if they're not, usually someone who could get that many kills was going to win by a landslide anyway)