Mine are people who use too many filler words in their communication. Such as "like", "kind of like", "Actually/Basically/Seriously', "You know", "You see", "I mean", "Hmm", "I guess", etc. Not saying all of these phrases are never relevant, such as "kind of like" or "like", when used in the proper context, but too often they are used where they have no meaning. If you listen to someone who uses these filler words all the time, in almost every sentence, it takes away from the point they are trying to make and gives the impression of insecure communication in some cases.
A good way to quit doing this is to pause where you would use a filler word. It makes for a more stronger communication and it is more pleasant to the ear. I know the use of filler words is not an indicator of intelligence, but you sound smarter if you limit their use. This is especially important if you need to sound professional in your job. Using too many filler words makes you sound unprofessional.