My list some from my mistakes and some threads on Audiogon.
1. Buying a speaker that claims to have very deep bass.
Take those bad boys home and whow! That bass is getting my room excited and is ruining the music. Setting expectations about how much bass is possible from a fairly inexpensive floor standing speaker. Expecting to much is a huge and mistake.
2. Not listening to or not being aware of your reactions to the sound. Ask your self am I relaxed while the system is playing or am I sitting on the edge of my chair. Is the sound involving or is it just a good PA System? Does the music evoke an emotional response? It should...
3. Is the sound cohesive. A cohesive sound will also sound pure and not blurred. Complex speakers can sound like complex speakers.
Those are my top three and yes I have kissed a few frogs..yuck!
Jim