good qualities include a good knowledge of spelling punctuation and grammar, (SPAG)
make sure you plan well. and don't meander around a point.
good essays are structured so that they have a clear and defined point, introductions, writing and end conclusions based on the information given in the main writing.
Here are somethings that are not appropriate and some good advice.
don't ever say billion, one in a billion, three billion or anything like that, in America a billion is 1,000,000,000 in Britain that's a milliard, a british billion is 1,000,000,000,000.
in fact whenever writing numbers write figures, not words.
if you're writing a number, use the internationally recognised scientific index.
eg, 1,000,000,000 should be writing 1x10^9
slag, just no...
be aware of the words that you're using:
if you say that something happens periodically, that implies a regular period of time.
there is only one best, two things can be better than something else, but they can't both be the best choice, or joint best.
that's all I can think of right now.