Nope there isn't any difference as such! I don't do it myself, but have known from friends that the Adsense performs better only if you have good traffic!
I have put AdSense Ads on my sites and my blogs. There is no need that you should register all your sites or blogs. If you have one 'pub-Id' that can be used on every sites and blogs. The income you are getting from different sites depends on the click through rate. So just put the code in multiple sites and blogs, then try to augment the traffic. That is all.
Nope there isn't any difference as such! I don't do it myself, but have known from friends that the Adsense performs better only if you have good traffic!
and if your traffic is from lots of different countries.
No , there is no difference, even if you are talking about having a blog on blogger.com. You can earn from adsense if you use adsense in a "proper way" and the rest is depend on the frequency of clicks, users do on your site.
Yeah I think there is no difference too, but it does matter on the keywords your blogs or domains have.
Some keyword ads are payed more than the others.
I don't think there any different in term of that, the different probably come from the Publisher, I think they set they price (do correct me if I'm wrong)