I know dese dere vidja gaems so let me throw in the same stuff everyone else already said (I actually know my schitt)
Preface: I don't HATE hate CoD. CoD is not an exceptionally bad franchise of games. It will never come close to Desert Bus or Big Rigs or E.T. or Lester the Unlikely awful. However, it IS incredibly tedious and shit with rehash after rehash. It's just as bad as Mario in that department. CoD went to complete shit after World at War, the fifth game.
-Old CoDs had "dated" graphics, but so do new CoDs. Every single new cod is the same or worse than the last. For example, Modern Warfare 3 is the exact same game with the exact same graphics as Modern Warfare 2, except it has DIFFERENT WATER TEXTURE. OOOOOOOOOOH. I've heard some fanboys say it's worse than MW2's. Other than that, it's the same definitive no-color-n-bloom game with shitty post-processing effects and engine age.
-Just gonna throw out there I think the HUD and UI is awful but when you replace a health bar with "B-B-B-BLOODY SCREEN, SO REEEAAAL" I guess you need to fill it with SOMETHING.
-CoD has had the exact same game mechanics and modes for just about the entire time of its existence. It's like the lay's chips of multiplayer, where it's very easy for you to want to keep playing. Ironically, for "run-n-gun-fun", it's insanely tedious and gets old incredibly quickly. Besides the basic modes you see in every game, they added ZOMBIES. Zombie mode was fun in WaW and then got boring when they included it again and again and again. Also last time I checked hit detection was awful, but I think I'm getting that mixed up with Counter-Strike. Prestiging is also worthless. Wipe your level and unlocks just to show off a new shiny badge. We'll get to that later.
-CoD's maps are shit. Seriously, they are really awful. Nuketown's a pretty good example of shit maps. Most of CoD's maps are somewhat medium sized or small maps. In CoD, small maps aren't fun. UT2k4 has large maps and THOSE are fun. Battlefield (and the much better Battlefront) have large maps, and THOSE are fun. Why can't CoD have good large maps? OH WAIT RUN-N-GUN "FUN" (The other games mentioned here have small maps but those aren't as bad as CoD's because the gameplay isn't awful.) I could go on but I would have to list specific maps and that could take all day. Even DOOM and Quake 1 completely blow any CoD the fuck out of the water in the map department. Community maps like achievement_turbo in TF2 are better than most of CoD's maps.
-The engine they use has been in use for a good while now, more than you'd think. And much like TF2, it's gotten bloated with useless bullshit after useless bullshit. Ghosts is a pretty good "breaking point" where even the fanboys realized CoD was pure shit, because it used the SAME DAMN ENGINE with the SAME DAMN GRAPHICS. The shit engine is one of the biggest gripes of the entire fanbase (the ones who have at least two brain cells to rub together) since the start of last gen, and leads into every single point I've listed so far. Apparently, the Metal Gear/GoW/Bulletstorm/etc. ripoff "Advanced" "Warfare" will be running "new tech".
-You've probably noticed I didn't touch on the singleplayer campaigns yet. Neither has anyone else. There's a reason nobody plays CoD for the singleplayer. Despite it not having "N00BS TO PWN LOL!!!!1" the campaigns are incredibly lackluster and not worth your time. They are also controversial. In the beginning of one of the games, you and your guys go into a russian airport with LMGs and kill a bunch of civilians. In another game, it starts off with a city being bombed (from your viewpoint.) To mention a much better game, Postal 2 is pretty controversial as well. However RWS intentionally made Postal 2 to be as offensive as possible and it's FUN, whereas I'm sure that the CoD devs (there's two teams of devs) just made "controversial" shit to get more attention. I would mention the games that these specific events are in but I've completely forgotten as all the campaigns blend together and I can't tell them apart. It's not as if the storytelling is anything special regardless of campaign.
-Now let's get into the meat and potatoes of the entire thing: The community is terrible. Minecraftfags, bronies, TF2fags, GMod fags, these are all awful people. CoDfags are terrible as well. They are the very definition of casual shitheads trying to fit in saying they've played REAL GAMES by saying they've played every CoD and got to tenth prestige. It's a babbyshit Xbox Live magnet. The community no matter the platform is filled with raging babbies and tryhards and trolls to poke at the fire. Doing a youtube search for "CoD trolling" or "angriest player on CoD" will probably get you close to a million results. "1v1 quickscope noscope 360 PM you angrily on Xbox Live" seems to be the best and most widely used tactic between players. People actually cry over this shit and beat their consoles and controllers. You would think it would be endless hilarity to continuously troll people by being good at the game or using weapons like the shitty Riot Shield, but it can get real grating when all you meet are fags that get angry over a CoD match. Back to prestiging, people actually pride themselves over getting a higher prestige. They believe it gives them self-worth on one of the most rehashed and terrible franchises in video game history. I'm not exactly exempt from playing for hours on end, but these people play day in and day out with the same stupid tedious shit on the same tedious maps and the same tedious screaming 12-year-olds, JUST so they can brag about having a higher prestige than their friends. God damn, I could write a full post just about the community itself if I actually cared enough to do so.
-This brings us to my final point and conclusion: Marketing. Call of Duty is one of the biggest mysteries of the Stock Market. People are scratching their heads as to why CoD is still selling well, much less AT ALL. It's because of the braindead community and the incredibly crafty marketing department. I swear they've gotten close to "BUY THIS GAME OR YOUR FRIENDS WILL LAUGH AT YOU BECAUSE THEY ALL HAVE IT" at least once. It's all about appealing to dudebros and "friends" and "You'll be the coolest kid on the block with the new Rehash of Doody!" Mountain Dew and Doritos aren't exactly a good combination and yet it's marketed heavily for no reason other than to sell copies of the game and the products themselves (last time I tried normal Dew it tasted close to piss so I don't know how that's supposed to be good with Cheeto Dust: The Nacho.) For everyone that doesn't like CoD there are at least two that do, it has swindled millions of people into learning some of the worst of multiplayer conduct and gameplay. Every game is marketed the exact same way with maybe one different gimmick in it whether it's original or not, and people STILL buy into this shit. I can definitely sense a heavy peer pressure vibe from all of this. The new CoD is marketed as being better than the others with new gameplay and new engine and new graphics. I think that's a load of shit, but for once I WANT to be proven wrong. I'm hoping that maybe, MAYBE it'll re-revolutionize CoD and redeem itself for over a decade of solid rehash and tripe... or maybe it'll be just as bad as all the rest which doesn't matter since the devs and EA get money anyway. Only time will tell.