Shattered Kingdoms

I know there is the favor text to shield types such as kite shield, buckler, round shield, heavy shield, tower shield. But do these actually determine how well the shield works. What actually determines the performance of shield?

The values the builder assigns to them.

Use the compare command to figure it out.

Type is a suggestion, not a bounds, on item properties.

As far as a serious answer goes:

The only thing that affects the effectiveness of a shield is material and whether or not it's the right size. Your skill with Shield Block will proc more deflections, stance will add or subtract from the procs of that skill, and AP/MP will reduce appropriate types of damage.

Keep in mind that adding AP runes to a shield will not actually result in more blocks, but only damage reduction done to the head/chest/belt area.

It's been said in the past that a high dex score and aiming 'mid' will increase your chances of procing a 'block', but I couldn't tell you with exact certainty if that's true.