That's one thing that I saw as increasing the complexity, the AC bonus.
Varying numbers slow down the game; many players have to stop and recalculate their math each time something changes. It's not a problem with every player, but in general it gets more pronounced as you have a) more modifiers, b) more variable modifiers (such as Power Attack), and c) a longer delay between the initation of the action and its impact. With 4E assuming a 5-PC party AND more monsters, it's quite possible there could be a longer delay between the paladin smiting and assigning his ally the AC bonus and the ally being attacked and needing to remember that AC bonus.
It seems to me that you'll have a lot of cases of this:
DM: Does a 26 hit you?
Player: Yep
DM: Okay, the creature rakes it claws across you for (rolls) 15 damage and 12 damage, 27 total, and give me two Fort saves.
Player: Wait, I forgot about the smite bonus, that makes my AC 28 this round. That was assigned to me, right?
Player 2: Right.
DM: Uhh, hold on. Okay, one claw still hits, 1 or 2 it's the 15 damage (rolls d4), yep, okay, 15, now give me that Fort save.
It's not game-killer, but it breaks the flow and it makes the game feel more complicated. Even very bright and experienced players are going to forget about this bonus now and then. I don't know if the concept adds enough value to the game to make it worth the complexity.