For weapons, the damage and knockback are increased by 131% at a skill level of 75, and there is no improvement after that, which means you do double damage at a skill level of around 57, which is significant.

Stamina usage is reduced by 1/3 at a skill level of 100, so only 1/6 at a skill level of 50. I, personally, don't consider that very significant.

I just noticed the "and knockback" part. In most cases I find knockback a disadvantage, but that's a personal preference.

I notice a pretty drastic difference in run and jump between my main, which usually is pretty close to max run, and a pretty high jump, and a new character. It almost seems unfair (and likely is) starting my main on a new world