There is evidence in lore of dragons existing and taking on mortal form. Those other things do not even exist in game, therefore that takes it into the realm of LORE IMPOSSIBLE. Whereas anyone can say, "Is it possible at some point in time in Azeroth that a dragon took on a mortal guise and got into some shenanigans?"
I've been watching the argument of this thread unfold, and while this isn't entirely subjective I think that the Dragon RP idea, while Lore Possible, takes far too many excuses and explanations to be worth it.
For example, someone argued a dragon wouldn't need to study a race/nation/culture they have already observed for thousands of years. The counterargument was that cultures change over time. Indeed they do, take the transition from High Elves to Blood Elves for example. That was a significant cultural change, but I'm nearly certain if any dragon flights found it interesting they would already have designated someone to observe the new culture and bring that information back to their flight. However, even this change in Quel'thalas might not be all that important from the perspective of dragons who live thousands of years. And if that's the case for major events in lore, what does that say about the day to day things occurring in the cities in the more or less static game world we play in?
Of course, that was just one example and by itself can be counter argued (again) and does little in proving any real point. But looking at the issue overall, my point is that more questions are raised than answers, and the subject has crossed into a very gray area between the black and white.
Rule of thumb, no matter what you play, is don't expect everyone to like your RP no matter how plain jane, middle of the road, awesome, or way out in the left field it is. Don't force them if they back off IC or OOCly. This virtual space is big enough that we can all share it without bothering one another. :)
This is very true, but is your goal for RP really about doing something challenging and showing it to others? That's almost synonymous with Palehoof's comment about 'roleplaying a dragon for the cool factor.' Why intentionally prevent yourself from roleplaying with a large chunk of the playerbase, when community interaction is so vital for our playstyle?
I'm not going to try and establish what people can and can't roleplay. But from what I've read here... I'd advise against roleplaying a dragon, there are too many complications.