But the whole of 4th won't make much sense if you base it on reality-
My rogue has coup de graced an unconsiouc humanoid, and failed to kill them. Now, as a medical student, I know that if someone with any kind of training tries to kill a helpless person with a dagger (such as by stabbing them through the eye, or slitting their throat), the chance of them surviving is zero, regardless of how tough they are [well tehnically they might survive being stabbed through the eye, assuming they get to a hospital pretty sharpish. They'll still be incapacitated though].
However this is jusy how 4th is. You cannot have long, complex combats, which are also realistically deadly. Compared to a coup de grace, a dragons breath is clearly less deadly: people can survive some pretty horrific burns, and there is no part of dragon folklore that says that anyone but a powerful warrior will be instantly killed by a dragon's breath.
As for dragons overall- Their pretty good solos, generally at least as powerful as their level suggests. The black dragon in particular is more deadly than it should be- I played against one with a party of 5 at 3rd level. Result- inevitable TPK (unless you run away, which the Ranger did). With a cloud of darkness for an effective +5 to all defences, and all sorts of nasty attacks with ongoing damage, our party could never expect to beat that thing.
In fact, has anyone here ever fought/run a dragon and found it to be lacking in 'beef'?