Interestingly enough, what you're asking about, OP, is something I've been thinking about these last few weeks, especially after seeing all sides of the COD debate start yelling at eachother.
There is indeed only so much you can do with Mario. and to be honest? I think he falls into the 'if it's not broken, why fix it?' category. Mario is Mario. He's always been simple, kinda plain, just friendly gaming in general. Always been easy enough for outsiders to pick up, but he packs juuuust enough depth to let a veteran sit down and loaf for a bit and have the smallest of challenges while smiling in memory of happier days when Mario blew his or her little mind.
The thing with CoD is that it's a current-gen brainchild. Noone EXPECTS Mario to be mind-blowingly different. Noone expects big-time gameplay changes. Not to mention it's basically a known fact that with a few exceptions, Mario games simply are just solid. Again, if it ain't broke, why fix it? If there's ONE THING Nintendo knows how to do, it's make a Mario game. They may not know when to take your money if you're offering it, (*cough*Rainfall*cough*) but they know how to make a Mario game and make it pretty well.
Modern Warfare? To my knowledge, and it seems the general opinion, the quality of the games can sometimes be in flux. One CoD game may be amazing, another may just be awful. Then it may go back to amazing, or it may just go into the middle-grounded purgatory of mediocre or okay. IW hasn't gotten CoD down to a science just yet. Without a solid gameplay experience to back it up, let's face it: visuals, it 'looking the same', is going to count.
Sure, in a perfect world NOONE should care about graphics/visuals. If the game is good the game is good. That's the bottom line. Minecraft shows us that much. Older graphics or a weaker overall Aesthetic can be countered or even negated by good gameplay. CoD doesn't always have good gameplay, apparently. Therefore, I guess in some minds, the least it can do is look good and be shiny.