Daniel Mendelsohn contends that to be a critic requires "expertise, authority, and taste." He leaves out the most important attribute a critic should have: the ability to pay attention. In fact, without this one, the others Mendelsohn mentions are superfluous
Any defensible judgment about a work of literature must arise from observable features on which the judgment is based and to which the critic can return. This is where the distinction between having an "opinion" about a text and being able to support that opinion is real. An opinion is only a provisional conclusion until it can be allied with and clarified by specific illustration from the work, until the critic can point to those particulars of the work that prompted the opinion. An unsupported opinion may or may not contain implicit but unstated illustration of this kind, but as long as it's unstated, it is not itself "criticism." Not everyone wants to be a critic, of course, but a book review, for example, can't really be taken seriously as criticism unless some text-based "evidence" is provided.
Providing such evidence requires that the critic pay attention. Close attention. This would involve, of course, noting, in fiction, such conventional elements as narrative structure (especially variations in narrative structure), character development (especially the writer's strategies for influencing our attitude toward characters), point of view, etc., but since fiction as a genre of literature is at its core the creation of illusions of such things as "character," "story," or "setting" through skillful manipulation of language, a critic needs ultimately to be able to focus on the writer's invocation of language, on the text as an artificial arrangement of words. Attempting to explicate a work of fiction by leaping first of all to plot or character or any other imposed device rather than considering the way such devices are conditioned by and embedded in language ignores the very medium through which literature exists, as if a work of fiction was really just like a movie aside from those pesky words. (Although film criticism certainly requires attention to the use of medium as well, in this case the manipulation of visual imagery.)
Being attentive to language does not mean picking out isolated passages of "fine writing" and making a fuss over them. More often not, such purported fine writing is just the decorative cover for a work that otherwise does aspire to be a movie. Ultimately language is everything in a work of literature, and a critic needs to account for the way a writer marshals the resources of language to create all of the effects in that work. If, for example, "setting" seems to play an especially important role in a novel or story, a critic should be expected to notice the way the writer's prose works to make setting (again an illusion created with words) seem so prominent. To a significant extent, this means the critic needs to describe the work at hand as carefully as possible, or at least the work as experienced by the critic (and potentially the reader) paying close attention. Judgment, which critics such as Mendelsohn want to assign such an essential part in literary criticism, can only be justified, and ultimately taken seriously, if it is preceded by this kind of scrupulous description.
Absent the effort to give close attention to the tangible features of the literary work, to explain what the experience of reading that work is like, what Mendelsohn calls "expertise" is largely beside the point. If Mendelsohn means by using this term to suggest that someone possessing it is an "expert" reader in that he/she does indeed know how to pay close attention, then of course I agree with him, although it is not necessary to be "expert" in some credentialed way in order to exercise this expertise. If, as I suspect, Mendelsohn means that a an authentic critic is one who can cite all the myriad books he/she has read, or has read all the right ones, or who possesses the appropriate academic pedigree, then this sort of expertise by itself is mostly meaningless. An amateur critic can read just as sensitively as a "professional" critic.
Indeed, the "authority" a critic can bring to the consideration of literary works can only come from the authority that the sensitivity and insight of any particular reading brings with it (although of course some critics can demonstrate over time a consistency of insight that gives that critic a kind of default authority). Unless the critic's work earns its authority, the sort of authority that comes from the supposed prestige of the publication in which that work appears or from some other external affiliation is just specious. Whenever someone like Mendelsohn (or Sir Peter Stothard, who recently opined about the "harm" blogging is doing to literature,) complain about the loss of "authority" being suffered by literary criticism (or book reviewing), it always seems to me they're basically lamenting the loss of this latter, artificial, and self-assumed authority.
"Taste," of course, is the most subjective of the qualities Mendelsohn prizes in a critic, and the purported possession of it by some (critics) and its absence in others (too many readers) has long been used as justification for the implicit deference we are to pay to the "best" critics. At some level it is undeniably good for a critic to be able to discern the artful from the meretricious, but the notion of taste is also used, frequently I think, as the excuse for bringing attention to some books and writers and ignoring others, thus giving the former the tacitly official approval of the guardians of literary culture. When critics are presuming to act as such guardians, their "judgment" especially needs to be examined with skepticism, as this act of sorting (abetted by editors) can actually do harm to literary culture by excluding adventurous writers and elevating those more acceptable to the cultural mainstream. "Taste" is again something that can be validated only by the strength of the critic's descriptions and analysis. It shouldn't be assumed.
I do not say that Daniel Mendelsohn in particular is guilty of this offense or that he abuses the critic's privileges in the other ways I have described. I find most of his reviews to be perfectly sound, although I don't always agree with him. And I do agree with him when he says that the "serious critic ultimately loves his subject more than he loves his reader." The literary critic's primary allegiance should be to literature, to its continuation and continued vitality. If his/her "expertise" consists of ideas about how to effect this, if "authority" is something that helps to ensure it, if "taste" means being able to recognize when a writer or work is likely to contribute to the effort, then indeed the critic needs all of these qualities.