Susan Sontag is "highbrow." Her essays have scope and intellectual ambition, which readers who have an allergy to these qualities may find "pretentious." She can almost mercilessly point out when something is derivative, weakly conceived, or a sell-out. She has a commitment to cinema as High Art; she takes the contemporary novel to task for being complacent and reactionary; she has a particularly sharp eye for intellectual fraud. Readers who are only interested in marching under one banner or another, or come equipped with biases or blind spots they are proud of, will probably find her annoying.
Sontag may be guilty of "neglecting to take into consideration" entertainment or commercial value, but I'm not sure why it necessarily is a requirement for her to take these things into consideration, since so many others are happily doing so. The fact that a film enjoyed great commercial value does not necessarily exempt it from being an example of "fascist aesthetics"; it simply may mean that it was a fantastically successful example of fascist aesthetics. Sontag was writing at a time when many used the word "fascism" in a very kneejerk way, as though it was this mysterious bad thing, an unknowable plague. Sontag doesn't allow herself such a simplistic attitude. She shows that in fact fascism has many attractive aspects, which is why its aesthetic still turns up everywhere, from Michael Jackson videos to Pink Floyd's The Wall to the WWF. I'm not sure she necessarily thinks this a bad thing; Americans, as we always like to remind the world, are free to enjoy whatever we enjoy, but at least we should not be dishonest about giving things their true names.
The judgement that this writer is a product of "1960s anti-establisment, feminist movement that views anything organized or male-oriented as fascist" is just a inaccurate, vague generalization whose purpose is to dismiss Sontag without having really read or thought about what she is saying. Sontag has skewered "anti-establishment types" and various feminists with the same lack of mercy she dispenses to Arthur Miller and Norman Mailer. Nobody's obligated to read Sontag or like the kind of criticism she practices. But for anyone really interested in cinema, art, theater, the novel, and related subjects, she's essential.