Which is the correct idiom:
Potential to vs. Potential for
Worry about vs. Worry over

Less than vs. Fewer
I understand that less than is used with mass nouns (non-countable) and fewer is used with countable nouns, but the following seem to be exceptions to the rule. Please Advise:

Its less than 20 dollars. – Arent dollars countable, thus why isnt it fewer than
He’s less than 6 feet tall. – Same, feet are countable, right?
Your essay should be 200 words or less. – Same logic.

I have the potential to go far. There is a potential for change. Either can be fine - depends on the construction.
I worry about her. You shouldn't worry over nothing. Again, either can be fine, depends on construction.

All three examples you list for the less than vs. fewer thing are items that are technically grammatically incorrect but they are so common in language that this is how people speak now. There are tons of examples of this in the language. Technically, they all should say fewer - it just sounds funny because no one actually speaks that way. :)

Stacey, how is "Its less than 20 dollars" incorrect?

Money is countable and fewer should have been used. But money is one of those things that falls in both realms: countable and uncountable. In this case. even though I know I have 'fewer' than 20, I don't exactly know how many and hence, I can't count. So, 'less than' is correct. What am I missing?

Stacey, to add, I found this from the Princeton Review book. I knew I had read something like that somewhere!

When it comes to money, you use fewer if you are referring to the number of actual bills you have, but less if you are referring to total amount and don't know the actual number of bills.

Right - the term "money" is uncountable, but the term "dollars" is countable, because "dollars" is supposed to refer to the actual, countable bills.

But "dollars" has become kind of fuzzy in the language - the term is often used in a similar manner to the uncountable term money, even though technically, it literally means dollar bills. I don't think I've seen the test specifically test countability with respect to the term "dollars" because of this reason.