Well, if nothing else this thread has given me some food for thought regarding the tactics, knowledge, motivation and puissant skill at arms of dragons, based on age and CR.
Take, for example, a Young black dragon...these things have an average of 85 hp, some decent abilities, and a CR of 4. If you play this dragon using "sound tactics," it will wipe the floor with a 3rd-4th level party. But take into account the dragon's Age, Intelligence, and Wisdom, and you've got a slightly less-formidable foe--perhaps a little overconfident. Like Khan, it might display evidence of two-dimensional thinking..."intelligent but not experienced."
In short, just as PCs should be able to draw a distinct line between what they know and what their characters know, so should the DM put himself in the monster's shoes and play them like a character, not a master tactician, IMO.
But everyone has their own style...I'm not knocking it.
As for the one-shot-kill, well..."sometimes you eat the bear, and sometimes the bear eats you." Any game based on dice will have lucky shots. e.g. 1st level fighter meets Great Wyrm. Hacks ineffectually at the wyrm's leg. Great Wyrm puts its head down there and says, "I'm going to eat you now, puny human." 1st level fighter makes last-ditch effort, rolls two 20s in a row, rams his sword through wyrm's eye into its brain: instant death. Not likely, but possible.
Again, what's good for the players should be good enough for the monsters. Instant death is a fact of D&D...it happens. DMs should always be ready for their "pet monsters" to die.
And...remove Polymorph Other? Come on...if you don't like the WotC explanation then make up your own. But wizards turning their foes into toads or newts ("I got better...") is a staple of fantasy and folklore.