What's your biggest sound quality frustration?

What drives you crazy about your system's sound quality, even after all the work you've put in?

• Different sound between driver/passenger seats?
• Issues with crossovers or system alignment?
• Narrow/poor stereo imaging?
• Something else entirely?

I've spent years in the pro audio world (concerts, theater) but exploring some processing techniques that might help with car audio. Not selling anything - just trying to understand what actual problems need solving before going further.

Thanks!