It does seem to me that, for listening to something very long, WTC or Goldberg or Art of Fugue or something like that, harpsichord gets pretty tiresome by about 30 mins or so in, in a way that piano does not. (Clavichord is probably a whole 'nother story.)
JZC - Maybe you've been listening to the wrong recordings? Hey, I get to use my education now - thanks.
Harpsichord registration changes allow a variety of sounds; although you can't make an individual note louder than its neighbors, you can use different combinations of sets of strings - but these changes in registration can be made only when there's an opportunity to do so (unless you have a pedal harpsichord, but most harpsichordists consider these inauthentic). I'll try to quickly summarize:
Italian harpsichords (of the era, and reproductions) tend to be the simplest and lightest. Generally they have only one keyboard (manual) and two sets of strings at the same pitch, called 8' (adopted from organ terminology) which can be used singly or coupled. 8' II will sound more "nasal" than 8' I because the string is plucked at a different distance from the end. (When coupled there's slightly more effort needed, because two jacks are being lifted at the ictus point rather than one; still less force than a piano key would need, though.)
At the other extreme are double-manual harpsichords, of the sort specified for the Goldberg variations. (It was very
unusual for any particular type of instrument to be specified in a keyboard work, and indeed the Art of the Fugue doesn't even specify keyboard; there are some nice recordings by chamber groups and even a saxophone quartet). A double offers a much wider range of possibilities; a typical disposition is 8' II on upper manual, 8' I on lower manual, and 4' (sounding an octave higher) on the upper, with optional lute stop (which presses bits of leather onto each string for a muting effect).
The two keyboards can be physically coupled such that playing the lower also plays the upper, but the upper can still be played alone. So someone playing the Goldberg variations can choose at a minimum from among the following:
8' I alone
8' II alone
8' II plus 4'
With keyboards coupled:
8' I plus 4'
8' I plus 8' II
8' I, 8' II, and 4' all sounding together
Note that whether or not the keyboards are coupled, a double allows one hand to play one registration on one keyboard, the other hand a different registration on the other.
So a double allows a lot of variety between pieces (such as the Goldberg variations) and even within pieces. This is not to say, however, that a double should be used for all harpsichord music.
(My Roland is single-manual but still offers 8' I, 8' II, 8' I + 8' II, 8' + 4', and lute stop.)