Still not seeing it all too much. First off, Portal is a puzzle FPS, not a platformer. Platformers never take place in first person. Look at Half-Life, plenty of puzzles in there. What do you do in Platformers? You run, jump, and get to different places. Not much different in theory.
Shooters have all guns a' blazing ones like Call of Duty. Then you have tactical shooters, like Rainbow Six, Ghost Recon, and the like. Do you shoot enemies? Yes. Can you run in and kill everything? Not even on the lowest difficulty. Your enemies can kill you with just a few hits, and you can't take them alone, hence your team. This makes them so wildly different from, say, Call of Duty. So different that fans of one may hate the other one for being unrealistic or simply for being realistic. Are you shooting enemies in both of them? Yes. Do they play the same? Oh, f*ck no! They play so different that it's just like the split between 2D and 3D platformers. Then take Portal, where you shoot things, walls mostly, and it's a puzzle that you get from point A to Point B. So much different from the others. Then you have stealth ones, like Hitman, Metro (if you play on the Ranger difficulties like I do, you have to use stealth), and Splinter Cell. May I also add shooter RPGs like Fallout and to an extent, the S.T.A.L.K.E.R. series (which isn't a full RPG, but has RPG elements). Not to mention survival ones like Fallout and S.T.A.L.K.E.R. Or the variation of scenarios in each game?
I fail to see more difference between platformers of the same type because you're still running, jumping, and getting from one place to another, collecting something, and so on. Honestly, they're exactly the same in the way that there are so many of them and they change certain things to make new ones. What's different from 2D and 3D platformers? Mainly the freedom of where you can go, but the concept's the same.