I think the whole concept of something being referred to as common is terribly old fashioned. It’s a class-riddled term and a way the upper classes used to maintain that they were superior.
There used to be lots of particular words were regarded as common; for example the upper classes would never use the word toilet, they’d refer to it as lavatory if they were to refer to it at all. Other words included serviette, which was one of the clearest indicators that someone had come from humble origins – the upper classes called it a napkin. And it wasn’t just about vocabulary.
"Flaunting your wealth makes it look as though you’ve only recently acquired money and that you’re not used to having it."
It was said that no gentleman would ever wear a brown suit and no lady would put the milk in her teacup before the tea (in the days before tea bags). The extremely upper class Nancy Mitford famously wrote about class hierarchies in the 1950s, but then with the social revolution of the 1960s things changed: people like the Beatles and the photographer David Bailey made it cool to come from unaristocratic backgrounds.
Most of us don’t care about traditional class distinctions any more: we’re more likely to use the word 'common' in jest. However, there are some things that it’s easy to describe as common or vulgar: flashy clothes and jewellery, say, or big TVs and entertainment systems. Flaunting your wealth makes it look as though you’ve only recently acquired money and that you’re not used to having it. Anything that is ostentatious could be frowned upon in today’s society. It’s much better to be elegant and understated.