Alright, it's not a secret that one of the reasons that the Fighter is
particularly bad is that Fighter feats don't scale very well. When you hit the end of one particular tree, you have to go back and start another one from the beginning. It's what makes the Fighter a really nice class to dip, though - you can pick up that one feat tree you really like, and to hell with the rest.
So why don't they scale well?
I think part of the reason is that, somewhere along the line, someone decided that the Fighter doesn't deserve to start with nice things. Note that I didn't say that they don't
get nice things - you have cool, fun stuff you can grab like Kharmic Strike, Improved Cleave, and Improved Trip, but you can't really
start with them.
So the question becomes "but do those powers scale"? My answer to that is
yes. Let's look at Improved Cleave - at 1st level, you're unlikely to be dealing a consistent amount of damage that would let you take out whole crowds. Normally, you're going to be petering out after one or two extra attacks. Flash-forward to level 8, where triggering the extra attacks is going to be more reliable simply because you're attacking more often and dealing more damage.
Another reason Improved Cleave scales is because it piggybacks off
attacking, rather than being it's own thing. As a result, Improved Cleave works with anything that makes a melee attack - compare that to Spring Attack, which falls behind
hard once you start getting more attacks in a Full Attack.
Improved Cleave also saves itself from obsolescence since it doesn't give you a static, numerical bonus. By giving you an attack,
it expands what you can do.
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The problem with the Fighter
in terms of why it lags behind other melee classes is because all it gets is feats. Feats that, may I remind you, don't scale, don't synergise, and don't do anything
fun. Heck, that's why mundane characters are kinda dull to play.
So why not rewrite the feats to be more interesting? Why not rewrite Spring Attack so it works off charging? Or Combat Expertise so it triggers off of Fighting Defensively? Hell, maybe even replace Weapon Focus with
Weapon Supremacy?