I read an article somewhere not long ago today that said Modern Warfare 3 had been criticised by the gaming press for having a campaign that's 'too linear'. Now, i've seen this crop up a lot lately, and it's become the latest punching bag in regards to FPS games. Remember the fuss about being able to get through the first level of Black Ops on normal without even having to fire a single shot? Not to mention internet celebrity Total Biscuit doing his damnedest to always rag on and on about not having any freedom in his FPS games, declaring it bullshit that he has to be told by the game when he can open the door, rather than giving him the free reign to just go and do what he wants when he wants. So if there'll be critical ire, it'll be for that reason - a campaign that's too linear and tries too hard to be a hollywood blockbuster. Though given the abysmal attempt from Battlefield to create a meaningful single player experience i'm willing to bet whatever we get will still be praised, supposedly 'wacky' story or not. Interestingly, i also read on the article that the single player won't matter too much anyway since the majority of the fanbase allegedly buys CoD for one reason only - multiplayer. I can't say either way how true that claim is, but it wouldn't surprise me given the high focus on multiplayer for the franchise, culminating in the Call of Duty Elite service. The multiplayer seems to be, from what i hear, virtually unchanged so people aren't going to get uppity about missing features and they'll likely be satisfied with just another iteration of the same thing. It's not so much about laziness as it is about managing your franchise and keeping a certain level of expectation satisfied.
As for the fans, they'll eat it up, no matter how good or bad it may objectively be. The fact one chap was willing to pay $1750 to get his hands on an early copy and a bunch of copies were stolen in a heist in France only goes to solidify this fact. The company i use to purchase games online also has notifications on your account page of any orders that may have to be processed early due to high demand. Unsurprisingly, they had a long message saying that Modern Warfare 3 was one of those titles and they'd have to start processing pre-orders early in order to just keep up and get everyone their copies by release. Even now, it's shown as being 'Unavailable' since it's out of stock. That's just one retailer; imagine how many will have a day one sellout, especially when you look at the number of people attending midnight releases. Regardless of what you or i may think about the Call of Duty fanbase, they love this franchise and i can't see it dying any time soon. In fact, this new zeitgeist of 'CoD-hate is cool' has only served to venerate this fandom. It's now the typical "CoD-kiddies" who look like the more mature lot, sticking to what they know and love while the opposition (like fans of Battlefield, for example) do their best to slag them off and feign superiority. That's not to say you don't get douchebags in the CoD fanbase, of course you do, but because of the nature of popularity the moral high-ground has shifted somewhat from the so-called "CoD-Haters" to the "CoD-Kiddies". If anything, they'll buy the next iteration of CoD and continue to do so if only to spite their critics.
I don't think the game will change dramatically. It'll still be the same old engine, but that's not necessarily a bad thing. It's extremely light and easy to optimise, meaning it can make games look good even on six year old hardware. Activision knows that the primary demographic for Call of Duty are console gamers, so they stick with the engine to squeeze out the best possible performance and still make it look good. (Incidentally this is also why you see a lot of games using the Unreal engine) I think Call of Duty's legacy will stand as a testament to the "if it ain't broke, don't fix it" philosophy and although it encourages stagnation, it goes to show that if you can create solid mechanics and keep a consistent presentation, it'll pay off.