I kinda like dragons not having polymorph. Having a bucket full of spells and a human form makes dragons into generic BBEG monsters; I'd rather see them as incredibly powerful beasts who don't NEED mortal-style spellcasting to meet their goals. (Heck, part of the appeal of a dragon as a craeture of pure greed is that they don't craft anything of their own, but rather take what they want from others.)
Someone said that it's easy to take away shapeshifting if it doesn't fit the campaign. I'd turn that around: if you want your dragon to be able to turn into a human, give it that ability. There's no reason that the "default" dragon should have a bunch abilities that don't directly relate to ripping up knights, burninating peasants, and sitting on large piles of treasure.
My random guess is that 4e chromatic dragons will be like this - brute forces of nature, wily and powerful - and metallic dragons will be the ones who play politics and deal directly with humans. So I can use my red dragon on a pile of gold, and you can use an iron/bronze/whatever dragon dressed up as a blacksmith in some intricate scheme.