Overuse of parentheses, by far.

Why is this a bad thing? Parentheses, as read by most readers, tend to contain additional superfluous information when read. So naturally, the mind tends to pay less attention to what's inside (or at least my mind does). I also believe it represents some repressed psychological trauma, since a Professor once circled how many times I abused the double dash, "--", and I haven't quite felt comfortable using it since--unless it feels right. As a result, I've found a new punctuation mark to abuse.

The parentheses and double dash work all right in that last paragraph, but relying on it leads to bad habits, and usually, sentences that droll on for far longer than welcome. Consider the following example from Naked Lunch:

Doc Browbeck was party inna second part. A retired abortionist and junk
pusher (he was a veterinarian actually) recalled to service during the
manpower shortage. Well, Doc had been in the hospital kitchen all
morning goosing the nurses and tanking up on coal gas and Klim -- and
just before the operation he sneaked a double shot of nutmeg to nerve
himself up.

(In England and especially in Edinburgh the citizens bubble coal gas
through Klim -- a horrible form of powdered milk tasting like rancid
chalk -- and pick up on the results. They hock everything to pay the
gas bill, and when the man comes around to shut it off for the
non-payment, you can hear their screams for miles. When a citizen is
sick from needing it he says "I got the klinks" or "That old stove
climbing up my back."

Nutmeg. I quote from the author's article on narcotic drugs in the
British Journal of Addiction (see Appendix): "Convicts and sailors
sometimes have recourse to nutmeg. About a tablespoon is swallowed
with water. Result vaguely similar to marijuana with side effects of
headache and nausea. There are a number of narcotics of the nutmeg
family in use among the Indians of South America. They are usually
administered by sniffing a dried powder of the plant. The medicine
men take these noxious substances and go into convulsive states. Their
twitchings and mutterings are thought to have prophetic significance.")

This is the only example I know that dares to put parentheses and a long quote within parentheses, unless we're counting math textbooks. In a way it works, since if information is ever superfluous, the history of sailors getting high on nutmeg fits that bill. A careful reader might easily pick up on that, but to everyone else, I think a full paragraph of text in parentheses signals the reader to scroll down in the hopes that the story continues.

But just in case you think I'm just picking on William S. Burroughs, here's an example I regrettably wrote to a coworker recently with some details removed:

[...] As a precaution, I did review $NOBODY's recent
$THING_I_WAS_ASKED_TO_REVIEW (if you would like a detailed report on
that, I can pull that together for you).

Somehow, I managed to take the one point worth emphasizing (an entire sentence at that!) and de-emphasized it.

It's a habit I can't break. So please, if you are a caring reader, do complain when I overuse parentheses. I deserve it.