The big advantage of heavy armor isn't net AC, it's that you don't have to invest in Dex. Since 4e assumes point-buy, that's a pretty big advantage.
If you're a fighter, you can dump Dex and Int to invest in Str and Con. If you're a cleric or paladin, you can invest in Wis and Cha. If you're a warlord, you can comfortably focus on just Str and Cha (if you're an Int warlord, you may as well go Light armor).
That's a big, big deal. You can keep up in Initiative with one feat (the "instant 18 Dex" of Improved Initiative), and your heavy shield gives you an effective 14 Dex in terms of reflex defense (+2 Reflex). With that feat and that piece of equipment, you can safely leave Dex at 10 and Int at 8.
So, for example, dragonborn paladin:
Str 18
Con 13
Dex 10
Int 8
Wis 14
Cha 16
Plate and heavy shield
AC 20
Fort 16
Ref 13
Will 14