Unlike the essays you may have written at school, writing at university level is expected to be reader-orientated and aware of its rhetorical situation, rather than an essentially writer-orientated display of knowledge. This means that nobody can give you a simple checklist of the differing expectations that apply at university level, because what is being marked by your tutors is often the structure and the cues you provide for the reader, rather than anything which can be reduced to discrete items. You can only understand these aspects of essay-writing by actively exploring writing strategies.
One of the fundamental differences between writing and speaking is the lack of interaction with the audience when you're writing, which makes it easy to forget to put design for a reader into your essay. The lack of audience interaction is also responsible for the feeling of not knowing what is expected which you may have. This is why it is often useful to give an oral presentation when working on an essay, since it helps you develop your sense of audience.
One source of information about what is expected in an essay are the published marking criteria for the module - although these are usually bullet points, you can ask your module tutor to explain them in more detail. It is also well worth looking over the feedback written on the coversheets of your previous essays - if you can spot a consistent theme running through tutors' comments, then that is obviously an area you need to pay special attention to when writing your next essay. Academic writing is often quite formulaic, and there are definite rules which can be followed to improve your work.