Since subwoofer placement was mentioned...
12a. Spending huge on electronics (Oh boy! That 5 - 45000Hz -/+3dB and 0.000000001% THD+N at 1kHz sure looks good on paper), yet, skimping on speakers ($1000 for bookshelf speakers is just way too much)
12b. Setting your very very very good bookshelf speakers (read, expensive) to large and not getting a subwoofer because the lower -3dB point is rated at 45Hz (and then wondering why they sound bass "lacking" at high volumes).
13a. Living in a glass house and complaining that the speakers are too "bright" or having super thick carpeted foors with triple layered thick drapes and saying the speakers are too "warm"
13b. Spending huge on speakers and/or electronics but, $0.00 on room treatments (at least get a Gramma if you are not in a basement situation to avoid the whole subfloor becoming a large highly inaccurate speaker).