This is a great website: Tips For Formal Writing. Dr. James A. Bednar has compiled this thorough list of things which will weaken your writing style, and possibly annoy your reader to the point where they are distracted.
Rather than re-invent the wheel, I’ll just quote my favourite part of the website:
Write what you mean, mean what you write
Speakers use many informal, colloquial phrases in casual conversation, usually intending to convey meanings other than what the words literally indicate. For instance, we often speak informally of “going the extra mile”, “at the end of the day”, “hard facts”, things being “crystal clear” or “pretty” convincing, someone “sticking to” a topic, readers being “turned off”, something “really” being the case, etc. Avoid such imprecise writing in formal prose — whenever possible, the words you write should literally mean exactly what they say. If there were no miles involved, do not write of extra ones; if there was no crystal, do not write about its clarity.
Among other benefits, avoiding such informal language will ensure that your meaning is obvious even to those who have not learned the currently popular idioms, such as those for whom English is a second language and those who might read your writing years from now or in another part of the world. Formal writing should be clear to as many people as possible, and its meaning should not depend on the whims of your local dialect of English. It is a permanent and public record of your ideas, and should mean precisely what you have written. - Dr. James A. Bednar
I truly appreciate the second paragraph of the quote. We’re still reading things which were written 3000 years ago. Would Aristotle have had the audacity to write everything down if he had known we’d still be reading his texts in this day and age? Perhaps you have no plans for your essay to ever see the light of day again, but you may not have such control. Take pity on the students who, 2000 years from now, may believe you to be an ancient expert on Edward Cullen. 
