While working on my BA in English, I helped a few engineering friends/classmates with their papers.
I don't mean to generalize, but one common fault I found in their writing was that they often put way too much thought into what they were writing.
Sure that sounds silly, but they just had an inability to write down information in a manner was easy to read. There was no flow. They're smart people and they often looked at writing as a simple task. Their attitude was usually something like, "I'm an engineer and this is easier than doing problem sets." That mindset held them back IMO. Often their papers read "like stereo instructions." They were full of information, but boring and difficult to sit through.
Mind you, the papers they were writing were for english classes, so they didn't have a whole lot of technical info in them. One classmate, in writing about Shakespeare's "Much Ado About Nothing," expanded the section on defining "nothing" to two full pages. The problem is that she could have made her point with one or two paragraphs at most.
These could have just been the issues with these particular people though. Again, I won't say it's something all engineers have a problem doing. But it was a common thread with the six or seven I had classes with and helped.