You should avoid:

Kitchen Sinking and Topic Jumping

Kitchen Sinking means throwing in all the complaints you have, large and small, not because they're on the same topic, but because once the floodgates were open, you couldn't stop. Topic Jumping is similar, and means attacking from all sides, like a pack of wild animals or a school of sharks, changing the topic to the next complaint before the listener can answer. An example of kitchen sinking is the letter that starts out "You hurt my feelings when you said...." but later changes the subject to a list of small gripes like "and by the way, we don't like your dog either."

Here's a good reason to stick to a single complaint topic: You want to convey (or at least not convey the opposite of) a belief that the relationship is salvageable. The problem with kitchen sinking and topic jumping, though, is that they convey "See what I've been thinking about you all along, while I was pretending to be friendly toward you?" So unless you want your reader to stop trusting you or to think the situation is hopeless, don't kitchen-sink. (next...)