I've worn armor, swung swords, shot arrows before, and know my history. Here are some observations:
1. Standing up after being knocked to the ground in plate armor is extremely difficult.
2. Heavy armor decreases the velocity you can swing your arm at, since your arm is heavier.
3. Firing an arrow from a closed helm: a) makes your aim less accurate since you can't get your eye in line with the arrow b) decreases the draw length, and therefore decreases the distance and damage produced by the arrow.
4. Wearing anything but smooth gloves seriously effects the release of an arrow.
5. It is amazingly hard to see out of a closed helm. Forget about lack of peripheral vision, you can barely see anything with some helms.
6. A common technique for killing knights is knocking them down and jabbing pointy objects between the plates, or into any openings.
7. A knight in heavy armor does effect the maneuverability of a horse. It's not the added weight, but mostly the shift in the center of mass.
8. You fall at the same speed with heavy armor as you do with light. Heavy armor does NOT cushion your fall. You're essentially falling onto metal.