At the 24-second mark in this Mercedes commercial (after some stunning images), a man informs us that the car has "less doors."
There are times when "less" may be appropriate with a count noun (for example, some people think "one less [count noun]" works, and I've seen more than one grammarian defend "10 items or less" signs), but the Mercedes commercial is not one of these times. It's not an idiom and "doors" has no underlying sense of being a mass noun. In simple cases in which you're choosing between "less" and "fewer," "fewer" is for count nouns (like "M&Ms," "doors," and "forks"), and "less" is for mass nouns (such as "water," "furniture," and "homework").
Mignon Fogarty is the author of Grammar Girl's 101 Words Every High School Graduate Needs to Know.