Fighters suck because:
1 - Their abilities don't scale. A spellcaster's power grows exponentially, a fighter's power is linear. Somewhere around level 5-6, spellcasters just pass up fighters on the power scale, and from then on, the differences just keep growing.
2 - Fighters are horrible defensively. Yup, you heard me, the grand master of AC is perhaps the worst defensive character in the game. "How so?" you might ask, well, I'll tell you.
a) Sucky saving throws. Let's face it, any spell requiring a will save will defeat the fighter any day of the week. Even if the fighter makes a concerted effort to bump up saving throws, he sill has a 50% chance (if he's lucky) of surviving all those save vs. horrible situation spells.
b) No buffing. Whereas spellcasters can choose to expend their spells to buff up if they know a big combat is coming up, a fighter can't raise his temporary strength at all.
c) No "oh-crap" abilities. Spellcasters can get the hell out of Dodge if the feces starts spraying, a fighter is just screwed. He's moving at 20" x3 a round, which is significantly slower than most monsters, so he can't really run. He can't teleport or dimension door, so that's out also. If he gets into a sticky situation, he'll have to fight like hell to get out of it, and if he fails, well, too bad.
d) If he gets grappled, then it's up to the party to save him, 'cause the fighter is screwed.
3 - Magic item dependancy. The four issues I just addressed can all be countered or alleviated using magic items, but that also means that if you take away a fighter's magic items, he's virtually useless. He becomes the party torchbearer.
4 - Absolutely no versatility. The narrow focus of feats (read: combat only) and absolutely horrid skill set (skills? What skills?) means that the fighter is absolutely useless outside of combat (where he's outshined anyways). In fact, he's a liability. Want to teleport? You gotta bring along the armored can too. Going swimming? You'll have to use spells to make sure can boy doesn't drown. Need to sneak into something? You have to use spells to make jingle boy silent.
The fighter achieves dominance over the battlefield until roughly level 5-6, after which he can usually at least contribute to combat until level 10 or so, after which he gets demoted to meat shield duty, a job he can't even perform well due to his horrible saving throws and lackluster hit points compared to a monster's damage output per round.