Luxury car:
High end materials, especially for the interior. Spacious, comfortable, and relatively quiet. Seating should be relaxing. Ride quality should be plush. Well appointed with options compared to its peers. Spacious luggage capacity for long trips.
Overall the focus is on a pleasant, relaxed, and luxurious driving/riding experience.
I know my own needs. And what I need from an automobile..
http://www.youtube.com/watch?v=Vsg97bxuJnc
Seriously though, I'd agree with Adrian & oldtin. The way I define "luxury car" is reduced NVH, through any means necessary (including telling the bean counters to back off while the engineering is being done).
In reply to Beer Baron:
Noise, vibration, harshness.
EDIT: Linkie. http://www.mts.com/en/products/application/ground-vehicles/full-vehicles/noise-vibration-harshness/index.htm
Low nvh levels, quality materials, bulletproof reliability. Think: Lexus LS460.
You know it's a luxury car when you step out after a road trip and feel fresh and relaxed.