I have to admit that I've sort of given up on this thread. However, a few remarks which seem to need to be said in relation to some of the more recent posts.
One of the key problems, it seems to me, that we are seeing in these posts relates to playing Bach -- or, for that matter, pretty much any baroque or renaissance music, but Bach is the most obvious target -- on an instrument -- a modern piano -- which he could not possibly have thought of. Depending on the particular piece, this may be easier or harder. In the case of pieces originally written for harpsichord, it isn't too bad; first place, all the fingering will be feasible. Second place, the tone of the instrument will decay with time; indeed, quite a bit faster than a piano's tone will. On the other hand, a harpsichord -- like an organ -- has discrete stops, not fine gradations of either tone colour or volume, and therefore changes in volume to bring out this line or that one are inappropriate (step changes in volume can be, but must be used with great care). What is appropriate, if one line is to be emphasized for some reason, is subtle changes in legato. This will be quite ample. The situation for the pieces for clavichord is somewhat similar.
However.
When we come to pieces written for organ, we are faced with a major problem: the organ tone does not decay, nor is there any possibility of changing the attack. You press the key or pedal, you have the note -- and it will stay that way (barring evil happenings to the instrument itself!) until you let go. Bach (and others) knew all about this, and meant it to be that way. You can't do that with a piano. If he wanted a sustained note over, say 15 measures or so, that's exactly what he wanted. No decay. No intermediate attacks. On the other hand, by using multiple manuals and the pedals with different registrations, you can get quite significant volume and tone colour differences, which you can use to great advantage (if also with great discretion). Bach knew all about that, too, and meant it to be that way (in fairness, he wasn't always very clear about registration and manual changes, but it's usually pretty obvious). To a limited extent you can do that on a piano, but only with volume, not tone colour.
For this reason, at the risk of sounding like a hopeless pedant (which I may be, I'll grant you) I have found that, with very very rare exceptions, piano transcriptions of works originally for organ are not particularly satisfying -- unless they were done by someone who was, in fact, a composer on their own, and essentially re-wrote the piece for piano, rather than simply transcribing it.
I am not suggesting that pianists should not play Bach. Indeed I would very very strongly recommend that they do; the Inventions, the French and English suites, the WTC for sure. But I have to admit that I would suggest staying away from the organ chorales, or the Orgelbuchlein, or any of the organ preludes and fugues, unless they are using one of the few really good transcriptions -- which are fiendishly difficult, even for pieces which aren't all that hard on the organ.
The situation is, of course, even worse for pieces originally written for voice or instruments...
Of course, it goes the other way, too -- compare, if you will, the Ravel orchestration (really a complete re-realisation) of the Mussourgsky Pictures at an Exhibition with the original piano work...