I’m not being snide. It’s about counting.
If you have items you can count, you need to use “fewer” to follow the standard usage guidelines. (What constitutes “standard” is up for much discussion, as any web search will readily confirm. I use it to mean “no one will look at you funny for using this, regardless of where you’re writing it.”) Those signs at the market that read “10 items or less” drive me batty, personally. I can count ten items. If I have fewer than 10, I’m okay. If I have eleven, I need to get in another line. (Unless the checker tells me it’s okay because there aren’t many people in the line.)
If you have an uncountable noun (like “intelligence” or “ability” or “music”), you need to use “less” to follow the standard guidelines. “John is less intelligent than Jake.” (John may have fewer IQ points. “Points” are countable. “Intelligence” is not.)
This job took less time than the last one.
This job took fewer hours than the last one.
“More” doesn’t have the same problem: You can have more time, and you can have more minutes. (English is fun, remember?)
Grammargeddon! 
I find that most students here start out writing about less rather than fewer children. This gives me rather disturbing images.
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Well, yes, but… I always take it to be elision. 12 items or less (than 12). If this is in your head, it sounds all right. (There’d be fewer in the sewer if they’d bought less at the start.)
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