The game system really does affect the "feel" of your campaign. That's why I enjoy Hero System for superheroing (although I realize not everyone here is a fan). Especially with 3rd edition, it was hard for your character to get killed, encouraging cinematic brawls and outrageous stunts. And the system really does allow you to create nearly any character you've seen in comics or can imagine.
Villians & Vigilantes was also fun but you were limited to the predefined (and randomly selected) powers. Also, since you were playing yourself with powers, that's how the game system felt: "normals with powers." You might have awesome cosmic energies at your command, but you'd still huff and puff climbing the stairs -- and a kid with a pea shooter might be able to take you out.
Palladium's superhero game, Heroes Unlimited, had a feel similar to V&V. Combat was surprisingly lethal to a Champions player. Unless you had the power Invulnerability you'd better have ducked when the bad guys started shooting. And the random power assignment could lead to some kooky character concepts. That's OK as long as you didn't have a particular character type in mind, but if you'd wanted to play an homage to your favorite scarlet speedster it could be disconcerting to roll up a Robocop clone with pink horns and a prehensile tail. One thing I did like was the scads of skills every Heroes Unlimited character was required to pick. Before 5th edtion, a Hero System character practically had to choose between having powers or having skills. It was nice, in Palladium, to be able to do something other than melt brick walls with your breath.
By the same token, the game system affected the campaign's "feel" when my friends and I played Traveller. We were inspired by the likes of Flash Gordon, Captain Kirk and Luke Skywalker but because combat was so lethal and earning credits so essential, our characters tended to sneak around like scheming Harry Mudds. We still had loads of fun, but the rules didn't encourage brash heroics.
Based on the discussion so far, it sounds like a BRP supers system would have a "feel" similar to Palladium or V&V. That's fine if you're portraying costumed crimefighters or pulp heroes armed with weird science gadgets. It might not work quite so well if you're wanting to play Superman or the Fantastic Four. I've personally never been fond of "grim and gritty" comics. If I want realism I'll read the newspaper. Superhero stories are supposed to help you escape reality.
