For many a car itself is luxury. Not having to walk form one place to another, in all sorts of weather conditions would mean for some people that they are living a better lifestyle.
Yet for some, a luxury car would not just be any car. It would have to be fitted with all the mod cons and the latest gadgets. Obviously these gadgets and devices will evolve as technology progresses as there will always be something new and improved available on the market.
Different levels of luxury are available in different levels of cars. An Aston Martin may prove to have a much more elaborate level of luxury than a Ford Feista, so luxury would be determined partly on what you are used to.
For example an Aston Martin maybe considered the high end sports cars and offer a level of luxury perhaps only paralleled by Bentley and Rolls Royce. You can expect such things as heated seats, and quick clear front and rear heated windscreens as well as handy iPod integration for those that like to listen to their own music on the move without having to transport a lot of CDs around.
Attention to detail could be considered as part of the luxury package. Hand stitched seat covers that can be detailed with your initials will also make the car that so much more bespoke as well as luxurious.
So it appears that luxury is down to past experiences, yet as you pay more you can expect to get a better level of mod cons and gadgets as the price increases.