Lets paint an image here.
Pick up that claymore. It's heavy. It hurts, give it a couple swings. It can hit hard, and has quite a lot of force to it, and it's very sharp.
Now, pick up that shield. You obviously cant swing the sword as hard unless you put the shield away, one handed swings are obviously weaker, and you need more time to recover. BUT, you are protected by a considerable amount.
Now put on that armor. Your flexibility and fluidity in your motions are severely decreased, also decreasing your performance in using your sword. So while you may now have a full suit of plate armor and a shield, your thrusts and swings are less effective.
This is exactly what this game needs. It should be a moderate reduction, but still a reduction. The tank stats should be increasingly relevant. Like 2k HP and 60 armor on one item with a 20% reduction on your damage on it.