Something writers think about, or should be, is word counts. In magazines, that seems to be laid out right in the guidelines. With novels, agents, and publishing houses, not so much. Colleen Lindsay has an update on expected word counts for genres, which seem to fluctuate. If you're shopping a novel, the whole article is worth a gander.
middle grade fiction = Anywhere from 25k to 40k, with the average at 35k
YA fiction = For mainstream YA, anywhere from about 45k to 80k; paranormal YA or YA fantasy can occasionally run as high as 120k but editors would prefer to see them stay below 100k. The second or third in a particularly bestselling series can go even higher. But it shouldn't be word count for the sake of word count.
paranormal romance = 85k to 100k
romance = 85k to 100k
category romance = 55k to 75k
cozy mysteries = 65k to 90k
horror = 80k to 100k
western = 80k to 100k (Keep in mind that almost no editors are buying Westerns these days.)
mysteries, thrillers and crime fiction = A newer category of light paranormal mysteries and hobby mysteries clock in at about 75k to 90k. Historical mysteries and noir can be a bit shorter, at 80k to 100k. Most other mystery/thriller/crime fiction falls right around the 90k to 100k mark.
mainstream/commercial fiction/thrillers = Depending upon the kind of fiction, this can vary: chick lit runs anywhere from 80k word to 100k words; literary fiction can run as high as 120k but lately there's been a trend toward more spare and elegant literary novels as short as 65k. Anything under 50k is usually considered a novella, which isn't something agents or editors ever want to see unless the editor has commissioned a short story collection. (Agent Kristin Nelson has a good post about writers querying about manuscripts that are too short.)
science fiction & fantasy = Here's where most writers seem to have problems. Most editors I've spoken to recently at major SF/F houses want books that fall into the higher end of the adult fiction you see above; a few of them told me that 100k words is the ideal manuscript size for good space opera or fantasy. For a truly spectacular epic fantasy, some editors will consider manuscripts over 120k but it would have to be something extraordinary. I know at least one editor I know likes his fantasy big and fat and around 180k. But he doesn't buy a lot at that size; it has to be astounding. (Read: Doesn't need much editing.) And regardless of the size, an editor will expect the author to to be able to pare it down even further before publication. To make this all a little easier, I broke it down even further below:
---> hard sf = 90k to 110k
---> space opera = 90k to 120k
---> epic/high/traditional/historical fantasy = 90k to 120k
---> contemporary fantasy = 90k to 100k
---> romantic SF = 85k to 100k
---> urban fantasy = 90k to 100k
---> new weird = 85k to 110k
---> slipstream = 80k to 100k
---> comic fantasy = 80k to 100k
---> everything else = 90k to 100k