Growing up, the most imposing critics were Bloom, Vendler, Hillis Miller, and Perloff, along with Jameson and Booth and a few others. I say growing up, because I was aware of at least one of these names even before I went to college. I subscribed to American Poetry Review as a teenager in the 1970s (not exactly a normal teenager, I know), and one day it came with a picture of Frank O'Hara on the cover, and an excerpt from Perloff's book Frank O'Hara: Poet Among Painters. F O'H was my favorite poet at the time, so I was very excited. Later, I figured out that it was unusual to treat him as a serious poet at all, in the context of academic literary criticism. Since then, I have been reading every Perloff book as it came out. The most significant ones, for me, have been The Poetics of Indeterminacy, The Vienna Paradox, Wittgenstein's Ladder, and The Futurist Moment. If I had gone into English, I would have been a minor Perloff disciple. Perhaps that is what I am anyway. My initial goal entering grad school in 1981 was to write The Poetics of Indeterminacy, but for Spanish and Latin American poetry. Not that book exactly, but something with that kind of impact.
The critical schools dominant in those days (early 80s) were avant-garde in theory, but had conservative aesthetic tastes. The idea was to do a really avant-garde deconstruction of a very canonical poem by Wordsworth or Keats or Wallace Stevens. Williams was considered still rather radical, so it was daring for J. Hillis Miller to catch up to him. Perloff changed all that by saying that Cage or O'Hara were worthy of critical attention. She was not "theoretical" in the same way, although she was (and is) theoretically astute in ways not always acknowledged. Her work met with a lot of resistance, but other critics ended up treating her poets seriously too, after a while. My generation was all Perloffian, because we accepted the legitimacy of Language Poetry and other forms of avant-garde poetics. At Stanford, when I was a graduate student, Denise Levertov opposed her hiring, simply because she thought the language poets were legit. She wrote a letter to the whole English faculty calling them "Gertrude Steinlets." Others continue to attack her even today. It still seems an affront to treat the language poets as serious poets and intellectuals, I guess, even though some of these poets have been active during parts of six decades.
Aside from deconstruction, the other dominant school, as I perceived it, was Marxism.
Marjorie, on the other hand, is a formalist and an empiricist. Her roots in formalism (not just quasi-formalist New Criticism) have given her a distinctive voice in criticism. Her first book was on rhyme in Yeats! (The New Critics weren't great prosodists, generally speaking. They were formalist in the sense that they wanted to see literature as autonomous, but not formalist in attention to the actual form of the poem.)
She is empiricist in the fact that she pays attention to the facts on the ground, rather than letting theory bulldoze a path of least resistance. It is good that she is not a Marxist or deconstructionist, because there should be one major voice apart from those otherwise dominant schools. There is a difference in seeing postmodernism as a symptom of "late capitalism," as Jameson did, and seeing postmodern works as having their own individuality, their own distinctiveness that is not co-terminous with ideology. It is Perloff who is the more historical critic, because she looks to document what happened in art and culture and not just impose a theory of history on the documentary record.
If you've read all of Perloff, as I have, you will notice repetitions of rhetorical strategies, of ideas, of modes of analysis. It would be hard to have written all those books and not have favorite strategies of analysis or critical moves. I've even read her early book on Yeats. Some of her books seem to be saying the same thing as other books, at bottom, though each one also brings a distinctive perspective not repeated in the others.
She has made major contributions to the study of Russian and Italian Futurism, Cage, Beckett, O'Hara, and Pound, among many other writers. Her native language is German, but she grew up in the US after escaping the Nazis in Vienna with her family as a child. She knows French, Russian, and Portuguese as well. She is perceived as being too limited in her taste, when actually she has a far huger range of interests than almost anyone else I can think of within the field of modern comparative poetics. I'm kind of glad she doesn't do Spanish too, because who could compete with that?
All this is a roundabout way of saying is that if you try to debate me about Perloff on facebook, I will destroy you.