Areas that show potential for improvement in our department:
Assertiveness.
How to show when you do and when you don't understand the conversation.
The importance and etiquette of interrupting in American communication.
suggested by Robert, support - personal interactions, pervasiveness - unknown
Proofreading.
Good writing is rewriting.
Nothing should ever be sent without at least one proofreading, even a quick email. Following this advice, we will still miss mistakes, but we will build the habit of proofreading and cut down on our more embarrassing errors. (Proofreading more than once should be reserved for jobs that need perfection; manuscript writing as opposed to quick emails.)
suggested by Robert, support - personal observation, pervasiveness - unknown
Proper used of articles.
In my opinion, this is the single greatest source of grammatical errors in written work by non-native speakers. The rules for articles are hard to grasp, especially if they simply don't exist in someone's native tongue.
suggested by Robert, support - proofreading work by masters and PhD level authors, pervasiveness - OMNIPRESENT