Now what you posit could be a valid way of doing things - making AC be more of a "Defense Class," which would make it more like Reflex. But I don't think that is how it works in any edition of D&D - unless I'm missing something with Next, and they're going with a AC/DR differentiation. I'm not opposed to that, but it would need to be across the board and would seem to add a layer of complexity which goes against the "simple core" design principle.
It's been both at times. At least since 3E. Where 3E had various ACs for different situations (touch AC, flatfooted AC, etc.), Next has only the one. And spells like Ray of Frost that would have been against Touch AC in 3E now just use the normal AC in 5E.
So, AC in Next is pretty much an all-inclusive defense trait. It's avoidance, armor, and everything all rolled into one.
edit: I guess I should add that I think the ACs of monsters across the board are probably too low, especially at higher levels, which is where you'd see the dragons. Even Asmodeus only has a 17 AC. The stated design intention, though, is to allow a big group of level 1 (or level 0?) NPCs be able to take down a dragon, with hit points being the major factor. I think that's fine for most monsters, but I think it should be harder at level 1 to hit the level 10 dragon. It shouldn't just be a matter of not being able to take off enough hit points. It should be a problem of not being able to get the blow to land in the right spot, and so on.
This also could be a problem with the fact that we never got to see monsters in anything close to a final draft. Easy-to-kill monsters were a talking point all through the playtest process, and WotC made their initial pass at them in the very last packet. So, hopefully, we'll see some more challenging things in the bestiary when the game actually releases along with some adventurer fodder like kobolds.
