The decrease in accuracy and inivtiative is normal, all piecez that adds armor decreases some of your stats.
But to understand de decrease in damage you have to know something:
They are three particulare melee skills:
One-handed
Twho-handed
With-shield
I take it you put points in One-handed. So the points you put in that skills are active if you wear a weapon in one hand... WITHOUT shield.
If you want to carry a weapon AND a shield, you have to put points in the With-shield skills.
If you wear a shield when having points in One-handed skill, you will lose all benefit of that skill.
They are also some One-handed Weapons that completely blacks out the shield slot in you inventory, so it becomes impossible to wear a shield.
Many players made that mistake in the beginning <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />.
