This is directed at the people asking "Why the hell are people making such a huge deal about this?", and even moreso at the people saying "You really have no good, justifiable reason to be upset". Dedicated servers offer more to the player than peer-to-peer networking can provide.
In a multiplayer game where one player is the host, reliability is a large concern. You can't expect the host's internet connection to remain stable throughout the duration of the session. If the host's connection begins to falter at all or perhaps somebody using the same internet connection decides to watch a video on youtube, every player begins to lag. Dedicated servers are independent machines that often have their own dedicated connection, meaning their connection is far less likely to experience issues.
Framerate is another concern, as the lower the framerate of the host machine, the less accurate things like hit detection will be (assuming the netcode is handled in a way to combat cheating and lag using a method such as Source's). A dedicated server doesn't have to perform any sort of video output, meaning that while a player may be experiencing 10 frames per second due to 4 smoke grenades going off in front of them, the server is keeping a nice steady framerate, as all it has to do is keep track of the grenades.
If you've played a game like Team Fortress 2 on the PC, you've undoubtedly noticed how many servers actually dedicate themselves to one specific thing. There are an incredible number of servers running 24/7 CP_Dustbowl, 2Fort, Arena, ETC. While Modern Warfare smartly allows the player to select what gametype they want to play, they can't do that with maps, they instead are at the mercy of chance and of the party they join. It's frustrating to never get to play your favorite map, or worse, to have the people you're playing with vote to have it skipped.
Host advantage. I'm going to assume you've heard that before. Whoever is host will never lag, and compared to what every other player is seeing, plays a few moments in the future. 50 milliseconds of lag means that for half a second, a host moving around a corner to shoot another player has half a second before that action even happens on his opponent's screen. Even if his opponent manages to shoot him after that delay, the message has to travel back to the host, which takes another 50 milliseconds.
What does this mean? It means that the host has roughly 1 second of invincibility against the other player. A dedicated server puts players on more equal footing.
The player's experience
Every once in a while, you might find a server that has a great connection, a lot of regular players who are all kick-ass and fun to play the game with, and the server's rules are exactly the way you think that they should be. A lot of game communities form around exactly this. Removing dedicated servers takes the player's ability to enter a game knowing that it will be an enjoyable experience with players they are used to playing with.
I am a PC gamer first and foremost, and I am seemingly in the minority when I say that I am the legitimate customer. I have never pirated games, and I have no intention of ever doing so. It seems that when people talk about Infinity Ward's decision to axe dedicated servers, the conversation moves to "This is how they are going to combat piracy" or something of that nature. Personally, I just don't care about pirates, they hold no value in my eyes, positive or negative, they simply are. Personally, I believe the only way to combat piracy is to simply give customers what they want, instead of pissing them off.