Well, something had to inspire me to post regardless of how crazily busy things are.

Megan McArdle asks:

The word mitzvah is often translated loosely as "good deed". Some things that observant Jews consider to be mitzvahs, however, would not ordinarily be classified by gentiles as "good deeds", such as saying certain prayers over food.

My question is this: does the reverse hold true? Are their things that could be classified as "good deeds", but that would not be mitzvahs?

The answer to Megan's question is actually a relatively simple "yes," because the premise is incorrect.

"Mitzva" is better translated as "commandment," not "good deed." (Loose translations...never trust 'em.) As a theological matter, Judaism makes no claims that the list of commandments that qualify as mitzvot (613 of them) is an all-encompassing list of all possible good deeds.

To make it more complicated, not all obligations observed by observant Jews are "mitzvot." Some may not meet certain criteria that must be met in order to be classified as one of the 613 "mitzvot." (Analyses of those criteria were the subject of many great rabbinic debates and scholarship about 800 years ago, and not everyone came up with the same list. Here's one version, which I haven't checked for accuracy.) Some are a lesser level of obligation, based on rabbinic decrees rather than explicit Biblical requirement - or, more technically and commonly, rabbinic extensions of biblical obligations. (Not all obligations are created equal. Think of it as a "first-level" obligation as opposed to a "second-level" obligation.)

Megan's example of food blessings is actually a pretty good one: most of them are not Biblical-level and thus do not rise to the level of "mitzvot," but they are still obligatory.

You may be asking: "well, aren't those second-level, rabbincally based obligations..less obligatory?" Sort of, as best illustrated with respect to Sabbath prohibitions: all prohibitions can be violated in situations of life-threatening danger, but you can take liberties with the "second-level" rabbinic prohibitions when faced with lesser levels of exigency. So you really need to know what is prohibited on which level in order to know what you can permit in what circumstances. Rabbinics isn't a full-time profession for nothing.

Finally, on a meta-level, the question of whether the universe of Jewish obligation (on all levels, not just the 613 "mitzvot") encompasses all conceivable "good deeds" is...a good question. The mysterious master of the "Four Questions" conspiracy once published an article on the subject, whose conclusion was essentially "It depends on how you define your terms." But it is at least conceivable that the answer is "no."

So Megan's question has a one-word answer: "Yes."