The second mistake that I see is sentences that are too long. This is often done for the best of reasons, in that the writers are trying to show their readers that their ideas, while important, should be tentatively stated, as there are points of view, important to others, that need to be taken into account, especially when it comes to making a categorical statement, though these should be made only infrequently. As you probably just noticed, long sentences tend to lose the reader. They interrupt the flow that the writer is trying to establish, because the reader has to reread the sentence to get the full meaning.
Writers use long sentences for several reasons. Good writers know that statements have to be qualified, and they try to do that in the same sentence. The problem arises when (as in my example sentence) the sentence contains several ideas. The writer tries to qualify each idea or statement in the sentence, increasing the length of the sentence. This causes the reader (and sometimes the writer) to lose the thread of the sentence.
To avoid this problem, make sure that your sentences are focused around one idea. Any necessary qualifiers in the sentence will then not unduly extend the length. Short sentences have that added advantage of maintaining your readers’ interest. Keeping your reader interested is more that half the battle in academic writing.
When I am asked about sentence construction, I use the following rule set.
1. Longer than 8 words: Probably needs a comma.
2. Longer than 15 words: Probably needs two comma or a semicolon.
3. Longer than 20 words: Probably needs to be two sentences.
It really is (mostly) that simple.
(I tried to write a sentence without a qualifier. The closest I could get, was to put the qualifier in parentheses. Qualifiers are a necessary part of out lives.)