Katie at A Constrained Vision is one of the most thoughtful bloggers about education. In this post, she discusses the perception of poor writing skills by college students. One element of the problem is that students aren't widely read, and so they don't become particularly good writers.
There is an element of truth in that, but I think the largest part of the problem is that students in college are writing with the wrong emphasis. There is no denying that many of the college students that I teach are extremely bright. As such, they have been encouraged from a very early age to "express" themselves. Writing for them is a very self-oriented process, as if it is a reply to the admonition, "Show us how smart you are."
This is acceptable for a student in grade school, but as a student matures, writing needs to become more about communication and less about expression per se. (This is true even if the purpose of writing is still for students to show us how smart they are.) Communication is oriented toward the needs of the audience, particularly the audience's need to be persuaded of something in order to change its mind. This is certainly the case every time I read a student's term paper, an article in a professional journal, or an opinion piece.
I think that if this transition were stressed earlier in a student's education, we would see better communicators in college. What to do about the poor communication skills of college students is a bit of a problem. No college administration wants to have to devote large amounts of resources to teaching a skill (as opposed to content) that should have been developed before the student arrived. In our current system, it ought to be taken care of with the 1-3 courses on writing in the first-year curriculum.