Here we can compare and see the consistency of the items based on what type they are, not necessarily based on their class.

For example for gems, Gem of True Sight is too small compared to other gems. We see the eye instead of the gem. It becomes more like an action icon instead of item icon.

For examples, the axe faces different directions. Maybe they can face the same direction for consistency.
Personally, I feel quelling blade looks too much like the orge axe.

For sticks, I prefer that all of them have the same orientation. So that at one glance, I know whether to look for differences. E.g. look at broadsword and claymore, given the same orientation with sword blade pointing down, I just to have look at the sword hilt to identify items. I don't have to struggle visually with the orientations.

For footwear, the most obvious one is Boot of Elvenskin which looks totally out of place.

Shields is a good example of a good icon set. We see the cheap stout shield looking plain while the more expensive items look more their worth. Lastly, the Shiva Guard with its bright and intricate background suggest power and luxury.

I observe a few colour associations.
Red = Strength
Green = Agility (or Health as in Healing Salve)
Blue = Intel
Purple = Invisibility / Stealth

This is most clearly seen in the Boots and Potions types. Should all items be based these colour associations? I feel no. But some form of easily identifiable consistency is needed. Maybe similar background subtle tones will do the trick. E.g. Power Threads