Pronouns (this, that, it, they, he, she…) have their place, but become incredibly irritating when overused in scientific writing and speech. When communicating your data, be wary of saying things like “This shows it is important for that pathway”. What shows? What is important? And which pathway?
Another phrase that gets me futzed up, yet never fails to rear its ugly head during talks: “They showed…” Who showed? Have you introduced who “they” are? While talks should be less formal than journal articles, you shouldn’t assume your audience knows who “they” are.
I also can’t stand writing that focuses on sounding pretty rather than being accurate. When writing for a scientific journal, be sure your gorgeous verbage doesn’t misdirect readers. I like interesting and flowy writing, but not at the cost of it making any sense whatsoever.