A correspondent of mine recently had this to say: I'm appalled at the increasing use of less when fewer would be more appropriate. I was taught that if you could count them (people at a meeting) you used "fewer"; if you couldn't count it (sugar) you used "less."
It seems that the trend is to use less for everything. ... I can't wrap myself around using "less" when "fewer" seems so right to me. She asked me to comment.
The traditional rule is indeed to use "fewer" with things that can be counted.