hi, i havent had much experience on harpsichords but i have had a few lessons and was going to be performing bach triple haprsichord concerto before i develeoped tendonitis and it all fell apart!
the main difference is harpsichords have no dynamic range, musicality is brought out through ornamentation, delaying of notes (rubato), holding over of notes, and extremely well played articulation, without these the harpsichord is nothing.... I would advise any pianist who wants to start the harps to have at least a few lessons with a good harpsichord teacher as the techniques are soo different to piano, You may also find yourself coming away like i did with a far greater understanding when you take your music back to the piano.
Another thing is that us pianists are very lucky in that our music is almost always written out for us, with harps you are pretty much left to your own devices with ornamentaion, articualtion etc etc and you alaso need to have a good grasp of figured bass
hope this answers your quesiton a little