There are two types of people standing in a supermarket check-out line; those who use the “10 items or less” aisle and worry about how many things they have in their trolley  ,and those who want to use a thick red marker pen to scribble out the word “less” and write “FEWER!!!” in large, capital letters.
As a long-time sufferer of prescriptivism – that terrible affliction where you can’t help feeling that there is a right and a wrong way to use a language – I have to admit I’m getting better at ignoring such things and adopt a Zen-like calm at the checkout, murmuring internal mantras to keep my blood pressure down. The trick is to take a little time analyzing just why “10 items or less” can be seen as “wrong.” And it’s all to do with the nature of nouns and counting.
When it comes to nouns, one of the ways to categorize them is as either count or mass nouns. A count noun is one that – not surprisingly – can be counted. You can have one button or two buttons; one banana or three bananas; one mongoose or 24 mongeese. OK, so that last one was a lie – it’s mongooses. The point is that the noun in question can be viewed as a discrete item and quantified.
The contrary is a mass noun, which refers to a thing that can vary in terms of quantity but you can’t really count it. You can have salt and then more salt; water and then more water; fun and then more fun.
Morphologically, count nouns typically add an -s to the end of the singular form of the noun whereas mass nouns stay the same. Some count nouns have irregular plural forms – hence the goose/geese distinction mentioned as few sentences ago – and a few don’t change at all, such as sheep, deer, and moose.
Now, just to make things more interesting, some adjectives that are used to pre-modify nouns don’t work with both mass and count nouns. This is the case with fewer and less, where the former works better with count nouns whereas the latter typically partners up with mass nouns. So you have less salt, less water, and less fun but fewer buttons, bananas, and mongooses.
Mass nouns can, in some situations, defect to the count noun camp, usually when the mass is in some way chopped up into smaller pieces. So if you have water poured into glasses, it’s perfectly normal to say things like, “There were several waters in the table.” Similarly, when visiting a bakery you might say, “There were lots of breads to choose from.” But in both these cases, the “countiness” is due to the fact that the mass has been artificially quantized.
Unlike words such as bread and water, which seem to spend most of their lives being mass nouns, or dog and cat, which sit squarely in the count corner, words like hassle appear to swing happily back and forth between mass and count. Thus having fewer hassles can, indeed, lead to less hassle. More specifically, the first hassle is in its count form whereas the second is the uncountable mass version. If you think about it, you can talk about having “lots of hassles” because you can in theory count each individual hassle, but if you’re talking about hassle in general, it’s a more amorphous mass of “hassle” so uncountable. If I suggested replacing that second hassle with harassment, the mass element becomes more obvious.
Now you get an idea why we poor prescriptivists suffer from bouts of toe curling when seeing “10 items or less.” It’s that the noun items is clearly a count noun (it takes an -s plural and is preceded by a number) but less is reserved for mass nouns.
 There’s a sub-group here of sociopaths who either cannot count and so trundle through with a cart positively overflowing with stuff, or are so egocentric and narcissistic that they couldn’t care less how many items they have – they just want the shortest and faster line so they can get on with their terribly important and self-centered lives. In the world of self-carry laws for gun owners, it’s a surprise that there are so few gunfights at the Walmart corral.
 We like to think that the average Speech Dudes reader is not, in fact, average, and is more curious than a clowder of cats, and as such, may ask the obvious question as to precisely why isn’t the plural of mongoose mongeese? It’s because the word mongoose doesn’t actually have anything to do with the word goose in the first place. If actually derives from the Portuguese mangus, which in turn is from the Indian dialect Marathi word mungus, and then ultimately the Dravidian language Telagu and the word mungisa. Any tendency to use mongeese therefore comes from mistakenly assuming it’s a derivative of goose, which comes from Old English gos and can be traced back to Old Aryan *ghans.