I grok that people may be concerned with young girls being "misled" into believing that the real world operates by Disney rules but I figure that any real problems would largely be the fault of parents pampering their little girls into such a mindset rather than Disney's fault.
I rather enjoy the way that Disney's modern princesses have (for the most part, I think) left the pure "Damsel in distress" theme fall to the wayside.
In the end the Disney princess themes are too pervasive for some people out there to not find fault with them if only because said people are tired of the prevalence of Disney princesses. That doesn't mean that people are wrong or right to nitpick Disney--everyone has a right to their own opinion--but I wouldn't place too much emphasis on the loud minority criticizing Disney Princess Inc.
That some people troll Disney princesses is rather in keeping with their theme, after all. ^_^
People who are concerned about Disney Princesses being too unrealistic for little girls are being a little bit condescending to them at the same time.
Short of schizophrenia, we do grow up to learn the difference between reality and fiction. Little girls don't turn 18 and suddenly think, "Wait, you mean I'm not really a princess" or whatever naïve notion people against princesses would pitch. Girls figure it out, they're not stupid. At least no dumber than boys.
But as children? Let us fantasize about being knights or princesses or rogues or whatever. This kind of rhetoric just serves to rob the magic out of childhood.
I don't like princesses (Disney or otherwise) for the same reason I don't like the spamming of pirates and ninjas at children, they are terribly inaccurate and simplified portrayals of historical groups. Just as unrealistic, simplified, and caricature-laden portrayals of "cowboys and Indians" altered the perceptions of these groups in harmful ways, I fear that the overuse and glorification of princesses, pirates, and ninjas could warp children's worldviews on these things. Particularly the glorification of outlaws, princesses have been around for a long time, but there is an argument to be made that girls being bombarded with BS depictions of princesses does adversely affect their self-esteem and psyche growing up.
...on the other hand, I do realize that this might be "overthinking it". After all, there's no South Park-esque misunderstanding of modern pirates because of Pirates of the Caribbean. Is it really so bad if a bunch of kids have a wildly inaccurate view of pirates from centuries ago? Is it really so bad if girls have their fictional princess fantasies, so long as they understand it's a fantasy when they're older and really aren't playing up the virtues of an aristocratically controlled society?
It's totally understandable as to why people might want to cosplay it, however, elaborate dresses and costumes and all. I just suppose my problem with Disney Princesses isn't so much how they are portrayed in their respective stories, but the fact that ALL of them MUST be portrayed as "Princesses", even the ones that aren't any kind of royalty. It's Princesses as a brand that bothers me, I suppose. I mean, Celestia from MLP is literally some sort of sun goddess and for some reason she's still referred to as a "Princess". The word has lost all meaning. Maybe that's a good thing, from my perspective.
I think a lot of it is the result of mixed messages on what people should allow their children to do. Give them freedom but if they show interest in THIS it's hurting the cause or whatever so don't let them do that.