Firstly, I'm not saying this game is a bad game. I am saying, in internet speak, that it sucks. That means that I do not much care for the game. I can however see some people do heart the game and I understand why. Anyway, back to the harsh truth. These are just 10 of the billions of ways the developers of the game actively removed the fun from the game.

Perks
As with the guns, you unlock these as you go along, giving people who have played for longer an unfair advantage over new players. I'll come back to that later. Even if everybody started with the same perks, they would still suck balls. When I stab somebody, I don't want to be killed because they pulled a grenade out their arse*. Somebody said for every perk there is an anti-perk, that's a lie. Perks are just there so somebody else, even if you awesomely knife them, can still beat you.
* "grenades out the arse" line courtesy of Chad Lakkis.

Unlocking Better Guns The More You Play
When I get beaten, I want that to be because of their skill, not because they have essentially used MMO style grinding to unlock the better guns. Don't start with the crap about those guns not being better. I had a long conversation with a guy who was on his 5th time up the ranks and he was bitching about the fact he hadn't been able to unlock the better guns. In 1v1 situations, the better guns will come out on top more often than not.

Not Matching By Skill Level
I don't want to be put in a room with a bunch of people who are just going to either stand in front of me whilst I shoot them in the face, or who can shoot me from across the map through 3 buildings using nothing but a pistol. I wasn't to play against people who are about on my level, I don't mind a bit of variation, that's always good, but not just random.

Modern Setting – Arabs and Russians are Bad
Regardless of political opinion, shooting aliens is more fun than shooting typical Faux News bad guys. I could go on about this, so I will. Why can't we mix it up? Just once? Something origenal? I just don't find it fun anymore.

No Option to Avoid Clans in Most Game Types
As somebody who doesn't have a giant clan of pro's to call upon when I play, when I get placed on a team of loaners like myself, against a clan, we just get wiped, it's not hard for them and it's not fun for us. If I could choose for each game type between clans allowed or clans not allowed, that would be nice.

Helicopter
Fuck Off.

Guns Kill Too Easily
When coming across an enemy, I like being given a choice. Do I shoot, change gun, run for cover? CoD4 allows none of this and it's you or them, down in part to aiming, who got the drop and which gun you have. Most kills occur in 2 shots or less. The time it takes to die is way to short to be able to make a tactical choice, making your deaths even more annoying.

Camping and Sniping is Actively Promoted
Even disregarding the Sniper Challenges, with the fact guns kill to easily comes people's instinct to hide, or camp as it's known in the business. Running into a room only to be instantly killed by the guy hiding in a corner, or sniped the second you go near a window might be your idea of fun, but if it is, you are wrong. This is not fun.

No Level Customisations
I have had more fun playing map variants in most of the games I have played over the past few years than playing the standard ones. Forge in Halo for example has generated some of the most awesome level ideas and really added to the fun to be had with the game. By cutting this out of the game, they not only limit creativity and replay value, but also prevent the game from growing or expanding.

No Split Screen Over XBOX Live
Games are fun to play with other people. You can play CoD4 split screen locally, but not online, why? Because that would be fun. One reason for this that CoD fans have told me is that using split screen would be unfair. Firstly to that I say “My good Sir/Madam, that's bullshit”. I back this up with the fact that there is almost nothing you can do online using split screen you can not do by simply playing with a friend as a group. I wouldn't even mind if they limited the number of game types you can play with split screen, but to take it out completely? Your cutting out a large section of fun right there.

The fun of CoD4 comes from winning, not from playing.

Also, if you like Call of Duty 4, this is a personnel attack on you and your opinions and I think CoD4 is the worse game ever and want to burn all copies. You should now either pick one point I made and rip it apart, or insult my mother.

:-P