Here's a few more from my list:

Answering "The right solution to this decision problem is X" (and seemly being satisfied with that) when the answer that's generally wanted is of the form "Y is the right decision theory, and here's why Y gives the right answers to this and other tricky decision problems".

Taking speculative ideas too seriously and trying to apply them to real life before the necessary details have been worked out.

Doing decision theory research might be a mistake in itself, if your goal is a positive Singularity and not advancing decision theory per se or solving interesting puzzles. (I had a philosophical interest in decision theory before I came to OB/LW. Cousin_it sees it mainly as a source of cool math problems. So both of us are excused. :)