My absolute top concern about D&D 4th Edition is how the effects of armour will be represented in the mechanics. I am partial to the "knight in shining armour" archetype, and I want it to be worth my while to wear my full plate all the way to 30th level. I don't want somebody who wears little to no armour and invests his character development into dexterity to be able to ignore more attacks than I can in my plate.
This could involve giving DR to armour, or allowing it to add a bonus to my fortitude/toughness save to resist damage in addition to adding to my AC, or adding "armour specialization" feat/talent trees so I can get more out of my armour, or simply ratcheting up the bonus armour grants to AC, or any number of other possibilities. I don't care which they pick, just as long as wearing heavy armour is rewarded rather than penalized at high levels. I feel armour got shafted in 3rd Edition, and want desperately to see this changed.