To varying extents, the formal features of genres establish the relationship between producers and interpreters. Alastair Fowler goes so far as to suggest that 'communication is impossible without the agreed codes of genre' . It's difficult to separate Mainstream pieces from Genre pieces - the mainstream's only a loosely related mass of more popular genres. There are even people who look upon each piece as a unique sub-genre, self-defining a way in which it can be read. Genres make life tricky for critiquers though. Pieces by Agatha Christie or Robbe-Grillet don't have character development - should we worry? Plot-driven works like Harry Potter use character development as a filler between action scenes - is that better? Gertrude Stein's repetition and restricted vocabulary are part of her game. Were Hemingway's later mannerisms as vital?
Once you're inside a genre, sub-genres appear, sometimes replicating the genres in the higher layer. Within Science Fiction for example there's hard-SF (written by highly qualified scientists), speculative-SF (idea-driven), comic-SF, space-opera, "1984", "The Handmaid's Tale", etc. Each sub-genre has implicit rules and priorities - it's easy to pluck a paragraph from Arthur C. Clarke's "2001: A Space Odyssey" and enrich its variety of structure and vocabulary, but doing so may well be counter-productive. There's no reason why expert scientists should make expert SF writers, though as readers they're best placed to offer the kind of factual critique which fans will appreciate much more than the rewriter's literary suggestions.
In this context where do criticisms like "sloppy writing" stand? Van Gogh's a terribly sloppy painter compared to Vermeer, and Vermeer's a mess compared to some Hyperrealists. And yet one doesn't want "it's a genre piece" to be a license for any type of writing ("so what if my story has a few spelling mistakes - so has Finnegan's Wake"). The genre competence of an experienced reader needs to be both nimble and creative. To readers who've read Fantasy and Realism but nothing in between, a piece of Magic Realism may seem an unfortunate mix falling between 2 stools.
Criticising a text might be tantamount to criticising the genre, and someone with no previous experience of the genre will have a hard job. Genre-sensitive criticism (aware of, but not subservient to, the demands of the genre) tries to relate the piece to its genre, using terms like
Because of this genre-related complication, some critics try to focus on factors which are common across genres. "Well-written prose" may not be a requirement for swashbuckling SF but surely there are some standards which apply to many genres. However, the value of elegant prose in itself is contestable. We're used to the idea that dialog can legitimately contain errors - after all, we're only human. The omniscient narrator is just another voice, and it's a genre issue (rather than one of quality or sophistication) as to whether that fact should be hidden. One can "show the working" - in a sense, all narrators are unreliable. A work's uniformity of narrative mode (or point of view) is also used as a supra-genre indicator of writerly skill, as is the writer's ability to make all readers think the same thing, or the author's ability to carry out his/her putative intentions. These too are genre-bound concepts though, considered ideologically suspect by some.
John Hartley argues that 'genres are agents of ideological closure - they limit the meaning-potential of a given text'  . Key psychological functions of genre are likely to include those shared by categorization generally - such as reducing complexity. This is a price we pay when trying to communicate, but genres can play a role in innovation too (in "The Name of the Rose", perhaps), providing familiar ground for readers to stand on while they come to terms with the unfamiliar.
New genres emerge from old much as new species emerge
As usual, once a reader strategy is identified, writers will exploit it. Readers look for categorisations that increase coherence and satisfaction. Writers such as Borges take advantage of this desire, leading the reader into one genre-context before surprising them with another. More radical genre experiments abound - "The effect which many identify with the Postmodern is produced by defeating readers' generic expectations" .