I also agree the resistance should be reduced and scaling, possibly like the tiefling's fire resistance. Otherwise, I think the class features are fairly good, assuming the Draconic Challenge still applies. I like the idea, though it does seem like a defender version of the beastmaster ranger, and have a few comments.
First of all, there is nothing thematically wrong with scale armor, as usually it's made of interlocking scales of metal, not literally scales. Actual armor made of scales (like dragon) would be strange, but that's a special case anyway. Automatic plate armor proficiency might be a bit much to start with; the fighter doesn't have it even though the paladin does.
Secondly, something to keep in mind for converting monster powers: there are no monster daily powers (I think, I could be wrong on this, but in general anyway). They are only expected to be in one combat, so a encounter power with no recharge may be as powerful as a daily, because that's as close as they get. This means when recharge powers are converted, they often become encounter, with encounter powers becoming dailies. There ore obviously some places this doesn't work at all, like dragon breath, but it does help sometimes, for example, suggesting the Frightful Presence ability should be at least a level 3 daily (or since there is none and it appears at higher level too, level 5, maybe with low damage).
Finally, if you're going with defender role and marking, you probably want about fighter hp. It's possible the dragon needs some changes to reflect being a defender too, especially if it also does some marking towards itself instead of you, but I'm not sure. You don't want to end up with too much hp, effectively adding another character's worth of damage absorption (unless that's part of the classes job as defender, but it would need to avoid being overpowered).
Anyway, looks like a good idea, just some things I noticed.