It's mostly just a case of having more scaling multipliers. All of the class "skill trees" (Warfare etc) are multiplicative with your damage whereas the weapon "skill trees" (Ranged, single handed, etc) are additive with your attribute damage. One point in strength and one point in single-handed or two-handed gives the same damage. Warfare is multiplicative so it's much better. Since polymorph gives attribute points its also worse than every other class skill tree.
Ranged attackers has access to three multiplicative skill trees in Warfare, Huntsman and Scoundrel, even if Warfare and Huntsman are probably better, especially Warfare. Every other class has access to less multipliers. Perhaps a mage could come close with the talent that allows spells to crit, their element of choice, scoundrel and Huntsman, but they have cooldowns with their spells anyway. Maybe dual wield wander can also get the same damage if they focus a single element with both wands and get the corresponding elemental class skill tree (it increases the damage from wands too, right?), and they can also get huntsman and scoundrel and crit.
Melee attackers have access to one less multiplier in that they can't get the "attack from high ground" multiplier, and thus they can't really scale as well.
I think changing weapon skill trees to multiplicative instead of additive along with changing Warfare to melee physical damage instead of general physical damage would buff everyone else to the Rangers levels. Maybe they would need to nerf damage overall if the game becomes too easy with those changes though. If they just want to nerf Rangers and keep everything else the same they just need to change Warfare to melee physical damage.
This is all from what I understand, and if I get something wrong then please correct me.