stats/skills

10 combat effects = +1% chance to proc

10 avoidance = -1% chance to be hit. This applies to the base % an attacker has to hit, so if attacker has a a 50% chance to hit, 10 avoidance makes it 49.5%. Calculated before attacker's accuracy. Don't believe this affects defensive skills like parry.

10 accuracy = +1% chance to hit, same rule as above i.e. 50% chance to hit becomes 50.5%

Note that there is a base % chance to hit, and a +/- % chance to hit based on level difference. The default values are 69% base chance and 1.2% per level gap, so you'll miss more on mobs which are higher level than you. Server admins can change this value. There is also a min chance to miss / hit which is written into the code. That doesn't mean it can't be changed though

Shielding is a little complicated, essentially there is a base damage amount which gets mitigated by shielding and aa's. 1 shielding reduces the base damage by 1%. Depending on the mob this can be a little or a lot. AA's also reduce overall dam.

Caps are server specific, but the base values are;
Accuracy - 150
Avoidance - 100
Combat effects - 100
Shielding - 35

AC is good, but AC can fail outright and not mitigate any damage. Spells/disciplines which reduce damage are always in effect, so warrior generally has a big advantage in tanking due to discs. They also have higher defensive skills than a rogue  In fact even a ranger has higher defensive skills than a rogue!

THF uses a lot of custom code so they may have tweaked things here and there, and changing the caps and base hit/miss values is as simple as updating a value in the database.