There are a couple of main differences between harpsichords and pianos.
First of all, the sound is totally different, because the underlying technique is different: in pianos, the string is hit by a “hammer” (well it is soft and not made of steel! but it is still a hammer), whereas in harpsichords (as well as other instruments such as clavicords, virginals and spinets) it is plucked by a plectrum (just like a mandolin or a banjo).
Then because the underlying mechanism is different, the touch of the keys is radically different: on pianos the fingers need to have the strength to make the hammers hit the string hard enough (and there is also a complex mechanism between the keys and hammers which makes it even harder), whereas in the harpsichord the key directly moves a piece on which the plectrum is and plucks the string directly, so the needed force is very limited.
Another effect of having different techniques is that whereas the piano is sensitive to how hard you hit a key (the hammer will hit the string harder and make a louder sound), the harpsichord has no or little sensitivity depending on the strength/speed of the hit on the key. In other words, on the piano volume dynamics are directly controlled by the strength of your fingers, not on the harpsichord.
All this has a very big impact on the playing skills of the artist, I was reading recently that you can compare the difference between a pianist and a harpsichordist with the difference between a tennis player and a table tennis player. The tennis player requires more strength, needs to run a lot, and these two very important skills are totally useless to the table tennis player. And on the other hand the table tennis player needs reflexes, extreme precision in both the aim and the effect given to the ball, which is of very limited importance in tennis. It is the same between the pianist and the harpsichordist. The piano player needs very strong fingers, sensitivity in the way he hits the keys, and these 2 skills are totally useless to the harpsichord player. On the other hand, the harpsichord player needs extreme precision to hit the keys (touching lightly a wrong key will trigger the sound, as strongly as the correct key you were aiming for, but on a piano it won’t have any effect), and the music often includes a lot of trills, which are not so common in piano music. And because the harpsichord has no dynamics in the keys, it forces the artist to adopt other techniques to make the music sound more expressive. Finally, the lack of sustain pedal can force the harpsichord player to hold keys to create the sustain effect.
To make a comparison, the hand of a harpsichord player is like a lace-maker, the hand of the pianist is more like a blacksmith.
Additionally, there are a few more notable differences:
-harpsichords may include several choirs of strings for each note, thus making it possible to vary the volume and tonal quality during the execution of a piece (to be compared with the organ registers), whereas the piano has very limited capabilities in changing the timbre (it has 3 strings per note, and on grand pianos it is possible to switch to 2 strings with a pedal action but that affects only the volume),
-keys are generally a little smaller on harpsichords than on pianos (that may be less than a millimeter per key, but it is enough to surprise and disconcert the pianist),
-harpsichords may have several keyboards (up to 3) which makes it possible to play the same note on 2 different keyboards (each keyboard is associated to a different set of strings), whereas pianos have only one, this can make some music very difficult or impossible to play on a piano where the hands would overlap because of the lack of other keyboards,
-pianos have pedals that don’t exist with harpsichords (including the sustain pedal, so on the harpsichord you can’t do sustain with a pedal),
-pianos have a greater notes range than harpsichords: 88 keys for the piano (that’s 7 and a half octaves), whereas harpsichords have from 56 to 61 notes (4 and a half to 5 octaves).
-the case of harpsichords is only made of wood, whereas modern pianos have a cast iron plate, which makes them much heavier (and of course has an impact on the sound as well).
-the overall volume of pianos is magnitudes stronger than harpsichords, this is related to the tension on the strings that allow very strong hammering that harpsichords strings would never sustain if they were hammered, so a grand piano can easily cover a full great concert hall, whereas harpsichords is limited to smaller concert halls. This in turn influences the repertoire: there are a lot of piano concertos since a grand piano can clearly compete volume-wise with a symphonic orchestra, whereas harpsichords are generally used as basso continuo in orchestras, there are some concertos but most of them use 2 to 4 harpsichords with a chamber orchestra since a harpsichord on its own has a lot of challenges to get a good volume balance with an orchestra.