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This is my take on the major innovations each studio has brought to multiplayer from Call of Duty 4 to Black Ops 2.
CoD4 had four major contributions to the series which now are so central to the game they don’t need explanation: the create-a-class system, perks within that system, killstreak rewards and the ranking up system with prestiging.
World at War for the most part copied the core elements that CoD4 had introduced with a few balance improvements like adding Flak Jacket to counter Fireworks and Shades and Gas Mask to counter tactical nades.
Two minor additions that World at War introduced: adding more create-a-class slots with certain prestige levels, and another layer of perks for vehicles.
Modern Warfare 2 expanded on two of the elements that CoD4 introduced: One is that you could rank up your perks to unlock a pro variant, and the other is that instead of a permanent 3, 5, 7 killstreak setup the game had an array of killstreaks from which you could select up to three, and some of those killstreaks were player-controlled like the AC130.
Black Ops did not have any major innovations to core gameplay. There was a lot of added personality like CoD points and contracts. To me it felt more different from Modern Warfare 2 than World at War felt from CoD4, but they kept the major changes introduced in Modern Warfare 2, specifically pro perks and selectable killstreaks some of which could be player controlled. What Black Ops did much better than Modern Warfare 2 was balance the game out.
A few minor additions that came with Black Ops was that killstreak kills didn’t contribute to your next streak reward, and some of the care package rewards were guns, powerful items that you could take with you during normal gameplay. Also Combat Training.
Modern Warfare 3 had two major additions to the series, Pointstreaks and Specialist. Up to this point only kills contributed to your next streak reward but in Modern Warfare 3 completing certain objectives also contributed to your next streak reward.
In my opinion the Specialist Strike Package is a major innovation whereas the Support Strike Package is not, it’s a minor addition to the game. I along with at least half of Call of Duty players has probably at least once thought how nice it would be if deaths didn’t cut off our killstreak, if we could get rewarded simply by getting a lot of kills in a game instead of having to get a lot of kills without dying.
Specialist however is rewarding a player who gets on a streak and stays on that streak with more character abilities instead of lethal rewards like a sentry gun or an airstrike.
Some minor things introduced in Modern Warfare 3, support strike package like I said before, a create a class tweak so that your streaks are tied to your class, another layer of perks that apply to your primary weapon which they called proficiencies, and streak rewards that wrap, so if you get all three rewards and then keep killing people you can get your killstreaks again even though you didn’t die.
Black Ops 2 is the first modern game by Treyarch that contributes major innovations to the series.
Scorestreaks. On the surface it looks like Scorestreaks is a tweaked version of Modern Warfare 3’s pointstreaks but the way I see it Scorestreaks are to Poinstreaks what Modern Warfare 2’s selectable Killstreaks are to Call of Duty 4’s permanent 3, 5, 7 setup. It’s a big enough leap forward to count as a major innovation even if it’s not a completely new idea.
The second major innovation is an overhaul to the create-a-class system which they called Pick-10, and I’m not going to try to explain it because you probably already know how it works.
Black Ops 2 has some minor but still significant contributions as well:
Black Ops 2 is also the first game to take a step back and simplify a bit. CoD4 introduced perks, Modern Warfare 2 put another layer in those perks that had to be unlocked through challenges, and Modern Warfare 3 added weapon perks (proficiencies) and split killstreaks into three strike packages. Black Ops 2 takes perks, the pro variant, proficiencies and weapon attachments, and reduces it all into simply perks and attachments which is exactly what we had back in CoD4. Black Ops 2 also didn’t continue with Modern Warfare 3’s strike packages, opting instead took the single killstreaks list from Modern Warfare 2 and Black Ops and converted it into Scorestreaks. But lest you think everything is simpler, they did add another layer of perks that directly affects how you can set up your class (Wildcards).
Another significant change is with the leveling up and prestige system, in that now when you prestige you don’t lose your progress in challenges and weapon leveling anymore.
Details on how perks and streaks have progressed through the series (warning: these are very disorganized):
CoD4 and WaW
MW2 and Black Ops
MW3 and BO2