The Meaning of Life, the Universe, and Everything.
I think this thread has many flaws, and I rarely criticize others threads. It takes too much valuable time to do so. But anyway, they simply would not fit in Minecraft anymore. At least the way you want. We do not need armor made from dragon scales or dragon teeth to make spears. The way I see it, if dragon armor was implemented it would use the dragon egg, but not consume it. EX. you would not lose the egg if you made armor out of it. And it was directly stated by mojang that they would not implement dragon riding.
As much as I love the idea of being able to ride dragons and burn all the villages down like a mighty god of fire and death, it seems more like something a person might put in a mod, if this were to be added to the game. As many people have suggested, for those that still support dragon-riding, perhaps adding a mechanic that makes you give up something of great value or great quantity to be able to do it. Whether it's gold, fire charges, or meat, I don't particularly care, so long as it helps balance things out.
I do like the idea of the dragons flying out and destroying villages near their lairs or pillaging them of their riches. Perhaps there could be a mini-quest that plays along with this, with the villagers pleading with you for assistance and sending you to do something about it. And perhaps an option for you to offer up any riches to the beast to temporarily appease it, because any dragon that's worth his salt knows better than to instantly vaporize potential treasure, especially if he thinks you could be of use to him in gathering more.
But with all this in mind, I don't really see how this could fit in vanilla Minecraft.
No support as a vanilla addition, but full support as a mod.