Why do all my professors want something different?
Faculty sometimes express dismay about what they take to be students' dismissals of their expectations about writing: once I figured out what she wanted, students sometimes say, it was easy... But there's some truth to that: the writing done in different disciplines really does vary considerably in objective, structure, style, evidence, and degree of formality. Different readers even understand what counts as "error" differently.
We recommend asking your professors for models of good scholarly writing, taking a look at scholarship in the field, even when your professor doesn't require it, and - as a simple way to begin to understand the sort of relativity of disciplinary discourse - consulting the Writing in the Disciplines section of Rules for Writers, the college's required writing handbook. We also highly recommend the really excellent disciplinary writing guides available at the Harvard Expos site, which offer a much fuller representation of different styles than Rules for Writers, as well as the archive of Dean's Writing Award papers in your discipline.
But of course ultimately, the best resource for a sense of the expectations of your professor is him or her: if you're at all unsure about this, always ask.