Just for an example, more loot//weapons// would fall more into the category of rpg content than anything else. You don't know what you are talking about. Just blindly defending without even reading what is being said. 2+2 is 4 you know. Not 3 and 1/2. You seem to have very crude understandings of what "rpg" elements means for a FPS//shooter. I would like to know what YOU actually think it means because you don't really seem to have a realistic understanding about what we're asking for or about what it really means to add rpg elements into a shooter. So please, elaborate on why you are against rpg elements into a shooter, and what you even think these "rpg" elements are or would be.
Well, first of all the OP wasn't just talking about loot and weapons, he was talking about leveling up and skills and stuff like that. Those might give some benefits, but when you add them you run into the problems mentioned before (using the same weapon type too much to level that skill/battle outcomes no longer depend on pure skill but stats as well etc).
For your specific post, adding more weapons wouldn't fall into the realm of RPG, that has nothing to do with it. What you are talking about is a Diablo or Borderlands like loot system. Those games might have millions of weapons, but they all belong to the same few basic weapon types. They have some basic categories like shotguns, machineguns, snipers, or long swords, bows etc, and just add a lot of stats to them. But playing with a low level machine gun, and the rare Uber Machinegun of Doom doesn't feel different at all from a gameplay perspective, you use them the same way, the latter just have better stats, deal more damage etc. So that alone wouldn't add more variety. Another problem is how you balance enemies around them, since devs won't know exactly what stats the players weapons/equipment will have. X enemy has Y hp and does z damage, usually these stats are balanced around the medium compared to what the player can have at that point. While you can be lucky and have an uber machinegun drop and go through them way too easily, or unlucky and don't have anything that does a lot of damage, in which case the enemies will be bullet sponges, and you will be abusing your left mouse button a lot.
Despite all these, I admit I like the loot stuff in Diablo, Torchlight, Borderlands etc. But i do not enjoy the combat in them. It gives me some satisfaction to see my character grow, and to have brutal weapons and all, but actually getting them isn't all that exciting. Its just a lot of mindless clicking. The combat in these games doesn't require either good reflexes/agility, or too much thought put into them. In a pure FPS if you can't get past a point, you need to aim better, and come up with a better plan of how to take down those enemies, which weapons should you use against which etc. Pure reflex based action and quick thinking that gives an adrenaline rush. If a game has a loot system, when you cant get past a point, you just go and grind some more till the game drops you more powerful weapons and armor. You don't need to be better at the game, only the stats of your weapons (or armor) has to be better.
These are two distinct game experiences, the first is closer to Doom, the second is closer to World of Warcraft. I'm not saying one is inherently better than the other, but they are different. Now, since Rage is an id game, who are known for making Doom and Quake it shouldn't be surprising that their fans expect something more similar to those and less similar to RPGs.
Adding a loot system, and other RPG like elements don't just add things to the game, they
change some basic things about the gameplay, not neccessarily for the better or worse (tho in the case of the combat I do think its for the worse, but that of course is subjective), but definitely towards a direction id software fans and FPS fans in general don't want to see this game go.