I'd have to agree with what Farm Zombie Again said above and [puts on literary critic pants] add that the knowledge of the history/background/theory related to the medium in question is a double-edged sword.
See, any kind of criticism – whether it's high-level academic stuff or simply the first impressions of a fan (which is not to suggest that academics cannot also be fans) – is an exercise in attention. The literary theorist Roland Barthes has a neat section in his book "The Pleasure of the Text," where he points out that pretty much nobody ever reads every part of every book. Sure, lots of people can with good reason say that they have "read" "War and Peace" or "Ulysses" or "In Search of Lost Time," but they probably haven't read literally every word. People skim, people have lapses in attention, and so your impression of a book (or movie, or whatever) is in part determined by what details your attention captured and what it didn't.
So the work of becoming a good critic involves a lot of practice to learn how to pay better attention, and a lot of the history/theory/etc works as a lens that directs what you draw your attention to.
Let's take a contentious example: the film "Birth of a Nation." Generally, people who comment on this film bring up two traits: its extreme racism and its pioneering cinematography. Both of these are relevant aspects of the film that are worth talking about, and which one you do talk about depends on the theoretical and historical model you're using as your lens. A critical race theorist and a historian of cinema will likely come away with completely different impressions of the film, simply because their attention was calibrated differently. (It's for this reason that I try to be at least familiar with a bunch of different critical paradigms at once – so I can triangulate between them.)
So the takeaway is that the academics and the fans are not as different as people often think. Everyone, it turns out, has a "literary theory," since everyone comes to a new work with a set of expectations and assumptions that will guide their attention. Even someone who rejects the work completely – someone who says "this isn't real art why even bother looking at it?!" – still has a theory that guides their reading (in this case, by shutting their attention off, or diverting it away). So this isn't so much a case of one or another reading being more or less valid, but rather a case of there always being competing versions of a given work existing in the collective consciousness. Works of art are, in a sense, created by their reception: the "Othello" you might watch today is different from the one performed in Shakespeare's time, and both are different from the one that was popular in the antebellum South.
This all becomes more complicated when we remember that there are different ways of enjoying a work of art, and different people seek out different kinds of stimulation from their media. I, for example, legitimately enjoy criticism – I experience it as an integral part of my reception of the work, and don't at all "get" (on an emotional level) the common aside that critique destroys the work. But someone who enjoys a work of art as a more instantaneous experience, a thoughtless rush of energy and stimulation, would probably find my favourite books and films quite dull.
Different works facilitate different kinds of responses, and the kinds of responses that a work causes (or, in some cases, demands) attach better or worse to different temperaments. And in the case of critics, often times the work of developing the faculties of attention that make proper critique possible also develops your ability to enjoy that critique. An analytic response to media can become more enjoyable the more your develop your skills at analysis. And so when you have started to practice and work on your ability to analyze, and have developed a taste for deploying that ability, coming across a work that doesn't facilitate it, or (worse!) one that actively scorns such a response (I'm looking at you Michael Bay) can be quite a let-down. The work feels hollow somehow, like a broken promise.
And this is one of the reasons why even the most detailed an analytic review of a work – at least at the level of the critic's affective response (that is, "I liked this!" or "I hated this!" or some such) – is always highly subjective, since it derives from this complex array of attention and expectation that can never be reduced to formula.
