The first thing wrong is that you are working on someone else's gear! (best avoided).
I've seen it enough times to suspect that the biggest problem is people buying a new TV, putting it into their system, and never going into the config menus. And, since most TVs come from the factory with the motion interpolation activated, the Soap Opera Effect runs rampant!
I think that people running their sub too hot is also very common (and I may be a bit guilty of this from time to time too).
Finally, the biggest problem that I see all the time is that people want good sound, but don't / won't spend the $ for it! I'm not even saying $1-2K. I'm saying getting people to spend more than $100 on a set of decent speakers is like pulling teeth!