I don't want to have to collect things to move on. I don't want to solve puzzles. I don't want skill points that make me better at shooting a rifle. What makes me better at shooting a rifle in a game is what makes me better in real life -- practice. When I started playing AA3 I sucked, I couldn't control my weapon, but with practice I got better. When I used to play SoF2 I would load a map locally and practice shooting walls for hours. Learning to shoot a perfect two shot burst at head level. People would think I was cheating because I could take their heads off before they even fired a shot. And it wasn't because of artificial skill points, it was because I practiced.
To me multiplayer FPSs are like competitive sports. They take practice, they are intense and thrilling, they require team tactics, there is competition against real people, and there is the thrill of victory from beating a real person. I don't want to out-smart code a developer wrote, or out-shoot an AI programmed to be less accurate than a computer should be. I want to out-smart a person who is trying to out-smart me. I want to out-shoot a person who is trying their best to out-shoot me.
I hope companies continue to make good SP FPS and RPG games, because I know people that really like them. They just don't do it for me.