"Less" for value, degree or amount; "fewer" for number

The Rule: Informally, "less" and "fewer" are interchangeable.
But formal English makes this distinction: "less" refers to value, degree, amount, or to abstract nouns; "fewer" refers to number, or the countable.

Informal: Less children are born during a depression.
Formal: Fewer children are born during a depression.


Harbrace's excellent Glossary of Usage doesn't note a comparable distinction between "above" and "more than," but in my mind there are cases (like dollar figures) in which "more than" seems more exact and other cases (as in percentages) in which "above" seems better.

So--
The city allotted more than $15 million to the new park.
Participation in voting was above 55 percent.