Typically PC games run a Dedicated server, and clients connect to them. Well, Infinity Ward in their infinite wisdom decided to axe all dedicated servers and replace them with a random player being selected as the "host", with all players connecting to them. The downfall to this is most servers are on 100+ megabit upload connections (really fast!), whereas the average user is on about a 1 megabit upload connection (about 100~ kilobytes per second).
This causes games to be laggy as home computers have higher latency (typically) than a server, along with slower connections.
Console gamers are used to this, so it's not such a travesty, but this is unheard of for PC games, so it's pretty much them stabbing PC gamers in the back so they can control the content and gameplay more closely (to charge for DLC, whereas in the past it ended up being free).