Academic writing requires that you draw on a variety of sources of information and make a judgement as to their authenticity.  The debate started by Abbi Williams on her blog (http://abbiwilliams.blogspot.com/2009/12/i-feel-little-debate-is-forming.html) about internet sources is a good starting point for this.  Academic writing places different values on information, and those values are always debated.  I would suggest using scholar.google.com as a good starting point to find very ‘academic’ style articles that can be then supported by information drawn from the internet or journals or books.  But here is a simple question…how many people checked the information you have used before it was published?  Anyone can publish on the net and say whatever they want, that’s the great democracy of the internet, but to be published means that other people have agreed with the accuracy, relevance or importance of the writing…

Simple feedback: draw on a variety of sources for your academic writing.  They may all be from the internet, but they are from a variety of sources, newspapers, journals, books, reports etc

A really good example of this use a variety of information sources comes from Laura Bentley http://laura-bentley.blogspot.com/

With marketing writing, some of the good examples I have seen have used a title to draw the reader in…

An opportunity not to be missed… (Laura Bentley)

You need to make sure that the purpose of your writing is clear.  Some of the academic pieces for example I am not sure what is the argument or question.  Once again, it may help to have a title or question at the top.  That purpose is central to the success of your writing.  A good example comes from Ciera http://cieramullaninteraction.blogspot.com/2009/11/writing-style-three.html

I keep coming back to audience…have you thought clearly about who the audience is for your writing? How do I involve the audience at the beginning through my heading and first sentence, how do I keep them interested, do they follow the story or argument and then at the end are they satisfied with outcome, leave them wanting more perhaps? 

In all three styles, some of the pieces I have read seem a little stop/start in that each paragraph is almost a separate piece of writing.  Make sure it reads a coherent piece of writing from top to tail and that each paragraph progresses and contributes towards your purpose.  A good example comes from Sophie Gilbert http://sophiejgilbert.blogspot.com/2009/11/writing-task-2.html