A few people have mentioned GURPS. As someone who plays both GURPS and D&D, I think I should make a comment here.
Both games have certain default styles of play (I'll leave rules modifications aside here - both games can be adjusted to fit any style of play... eventually).
In GURPS, no matter how good you will get stupid mistakes can still kill you. While a high-point fighter could easily cut through large numbers of opponents one at a time, if he is surrounded he is in deep trouble. You will have to retain some sense of tactics, or you will die.
In D&D at some point lower-level opponents will simply no longer pose a threat to you. They won't be able to hit you - and even if they did, they rarely do enough damage to threaten you.
Paradoxically, it is easier to die in D&D than GURPS if you face equally skilled opponents. In GURPS it is far more likely that you fall unconscious tha that you will die in combat. -10 hit points, on the other hand, are easily reached in D&D. Then again, it is often awfully easy to get ressurected in D&D...
And nothing of this has anything to do with those games' maturity, or lack thereof. Whether role-playing is mature or not depends more on the gamers than the game itself (though this theory obviously has its limits - view SenZar and its ilk, for example...)
In the end, our group plays different RPGs when we are in different moods. We play GURPS when we want to keep combat at least approximately believable. We play Shadowrun if we want to shoot things with assault rifles and level buildings with C-12.
And we play D&D if we want to see a few nobodies transform into mighty heroes who could stop whole armies without any help.