Actually it isn't Rossi that turns people off him as in general he is quite a likable guy....it is his more rabid fans that have damaged his image more than he could ever do. He does, however, have a dark side to his nature and this tends to be exposed when the results aren't forthcoming - as can be seen this year with all the recent jibes he has been making about Lorenzo, Stoner and all the other ........
Casey on the other hand has always had a tendency to pull no punches with his opinions and obviously has far less skill with the media than Valentino...he is very similar in personality to Michael Doohan in this respect and it is probably an Australian trait that some find confronting...many Aussies tend to call things as they see them and that does not sit well with many people.
I also think a lot of todays events (I include all current affairs in this criteria) are blown out of all proportion - the media like to stir things up and whip opinion into a frenzy....its good business.
Also with the 24/7 nature of news and the evolution of social media (blogging, forums, twitter, facebook etc) every single one of us is suddenly an analyst with an opinion on offer....this is unprecedented in human history and I am afraid many people lack the skills required in this area to make good judgement calls. Hence you tend to see histrionics, over reaction and appalling research skills replacing the moderate, carefully considered and insightful reporting that has traditionally been the domain of professional writers/reporters. My father is a multi-award winning writer/journalist in his chosen field but he just doesn't fit in or gel with todays style of media...he just isn't controversial enough and sadly his style is a dying art seemingly replaced with hyperbole and vitriol.
Show me a single rider in MotoGP these days who isn't criticized, if not crucified, often completely unfairly for behaviour which in the past wouldn't have even rated a mention.