The great sword wielder does not need to have as reliable damage as a dual wielder to be competitive. If he dies, that's great, that means the game is working. Low level PCs should be fragile and possibly die. If you give up defense to go all out offense, you should expect that it would negatively impact survivability. That way, a good defense is a good defense, meaning, one does not need system mastery to realize, unlike in 4th edition, that raw DPR is not the be-all, end-all of everything, and if you want to survive you should put some thought into defense. Maybe use a longsword and shield, and drop the shield when you're fighting less enemies, or ones with lesser fighting ability. This introduces tactics that makes the game interesting. PCs should not be guaranteed to win, and if they drop, they should have their party there to help them back on their feet.
What you described is the game working as intended. Low level characters shouldn't suck, but they shouldn't be particularly good either. And a first level fighter with a great weapon is still a menace.