I know the question's not for me, but here's my two cents:
Djokovic's image: the ironman of tennis, a tough competitor, warrior, gladiator who will leave everything he has while on battle, and yet he can be funny, warm, playful and charismatic off court.
His story is completely inspirational: how he triumphed by working, dreaming and never giving up
Natural demographic: young people with a drive to succeed, he should be a much more suited role-model for American kids than Roger or Rafa: he's the quintessential self-made man.
I've seen it here in Mexico: every young tennis player wants to be like Novak, they want to be the winners of fierce battles, they love his roars and passion. Novak represents youth and power.
That's for now...in the future, if he keeps being at the top of the game, his image will change.
And I hate to state the obvious, but in real life, Novak is the most sophisticated of the top 4. Hands down. Rafa lives with his family and doesn't speak good English, Andy is too introverted, Roger is just a famous guy obsessed with fashion and celebrities...Novak is much more educated and sophisticated in thought, his language skills are amazing, I saw him interacting with Cameron and with Prince William. He's a natural charmer.
His commercial disadvantage has been the fact that he's Eastern European, if he were American, Canadian or British, with those results and that personality, he would already be at the top of the heap... But he will overcome that disadvantage with his reslts and his charm: he's already doing it.
As usual you said it all and perfectly.
I also do believe, because I have seen it, Novak's appeal is very strong in not so developed countries or non-white countries. He has a very strong following in south america, arabia and asia, much more than Nadal ever did (not sure about Federer). He's also the player of choice for young kids these days, who soon enough will see Federer as a relic.
How can a guy with such an original background and story be generic? he has the good-looks, the charm and the charisma. Yeah, maybe his image is clean-cut in that he doesn't have long hair or tattoos or big muscles... but i think his image works better than Nadal's because he can sell a luxury brand and at the same time a youthful rebel brand. He's inbetween Rafa and Roger in that sense. He's also cultured, speaks many languages and has a clean image (a beautiful and loyal girlfriend, strong family values, inspirational story, etc.). I do think his big potential hasn't been tapped yet.
And by the way if it's an indication of anything, he's been on many fashion magazines in the last two years... Vogue, Elle, Marie Claire, also on the cover of Red Bull Magazine, GQ, The Men, Haute Living, L'Uomo Vogue...