I'm not nearly as impressed as I expected to be when I saw the thread title. :\
The things I liked are pretty standard: dragons are awesome, they have high AC, heaps o' hit points, and a single one has enough mojo to handle a party, unlike many other monsters.
The things I didn't like:
"It’s the fighter’s turn. He charges the dragon and manages to land a solid blow, dropping the dragon down below half its hit points. Oh—that gives the dragon the opportunity use its breath weapon as an immediate action."
OK, neat, but why is this?
I like intricate mechanics, so I'm not against the concept of conditional actions as such. But I don't like when the world seems like it functions according to the game mechanics, rather than the game mechanics modelling the world. What's the reason that getting hit lets the dragon use the breath weapon,
other than the fact that he has "use breath weapon when hit" ability?
For example, the other immediate ability mentioned makes much more sense:
"Now the rogue moves around to flank with the fighter. Ordinarily, that would let the dragon use its tail slap again as an immediate action, but the dragon has used its immediate action already."
Attacks of opportunity gone, replaced by specific abilities that let you take out of turn actions against people who, let us say, provoke you?

In any case, unlike the breath weapon thing, it's intuitively clear why circling a giant lizard would let him tail slap "for free".
He blasts the dragon with a ray of freezing cold, but this isn’t 3rd Edition.
Meh. I liked the dragon vulnerabilities. To some extent, it made them more vulnerable to metagaming, but really, would it be unreasonable for the character to assume that a white, ice-dwelling, cold-breathing creature was vulnerable to fire, even if he knew nothing about white dragons? What's the point of different energy types if they all work the same? Different visuals for the computer screen?
The cleric's heal-on-hit ability sounds like one of the Devoted Spirit effects from Bo9S. It's cool, but again, I'd prefer it if it were described in terms of in-game effect, rather than pure mechanics. "The wizard gets a second wind as he sees his ally defy the dragon." or something.