I've been thinking about this for a while and finally decided it was worth writing about...
The technique required to play Bach on the piano is completely different to the technique required to play, say, Chopin or Rachmaninov. There are some obvious reasons for this: pianos as we know them didn't exist in Bach's time (even during the classical period they were relatively new and very different to modern pianos) and fingerings were very different to what they are today - for example, the thumb was hardly ever used!
I consider this last point to be all-important. It goes a long way towards explaining why even the most technically proficient pianists often still struggle with early repertoire, despite being able to breeze through far more complex 19th and 20th century pieces without any difficulty. It probably also explains my own technical battle with Bach's music.
I tend to think of the development of piano technique as we know it today starting with Chopin; however, there were still composers writing for the piano in a relatively conservative style during Chopin's lifetime (during the first half of the 19th century). Chopin was innovative in many respects, and his 24 etudes in particular bear testimony to this, counting among the many works which are especially suited to playing
on a modern piano, with more or less 'modern piano technique'. Such pieces are commonly referred to as highly pianistic. A composer I consider to have attained the pinnacle of 'pianisticness' (no, that's not a word, I just made it up) is Rachmaninov, and any reasonably proficient pianist who has played his music will understand why.
There are ingrained fingerings that modern pianists tend to fall back on when sight-reading or playing a piece they haven't learnt intensively. These are the fingerings that occur most frequently in romantic-era piano music, and which therefore become automatic after someone has been studying the piano for long enough. Even when the pianist has memorised an entire sonata or concerto they are still likely to be falling back on these 'defaults' a lot of the time, without even realising. This is especially true in highly pianistic music, where the expected 'defaults' are rarely broken.
Fingering defaults are perfectly adequate for a lot of piano music written in the 19th century, and for simpler music of other eras. However, they begin to fail when the music doesn't fit into the common spacial patterns the defaults were designed for. A lot of baroque and classical music doesn't fit in; neither does a lot of 20th century music. In fact, there's a relatively narrow range of music in which the pianist can rely on these generic fingerings to get by. Extra work is required to be able to play pieces that fall outside the range of fingering defaults convincingly.
Since an enourmous amount of work is required to get a piece to performance standard anyway, this shouldn't be a problem. However, I've discovered that for me, at least, there are some issues with memorising pieces in which I can't rely on fingering defaults.
Some really unusual (by today's standards) fingerings are required to play Bach's music on the piano. In my score of his D minor concerto, almost every note is fingered. It's absolutely necessary; I would get totally lost without having meticulously figured out the fingering for every passage. Standard fingerings are useless in Bach's music, and even more so in his highly contrapuntal works (i.e. fugues) than in the relatively simple, mostly 2-part texture of the D minor concerto.
I've always learned new pieces of Bach using the 'metronome increment' technique: perfect a passage at a certain BPM, increment the metronome by a small amount, perfect the passage at the new speed, and so on until I can play it perfectly at performance speed. This amount of repetition, combined with the sheer oddness of the fingerings required, have always enabled me to memorise Bach very quickly. However, until I noticed how quickly I forgot pieces of Bach that I thought I'd memorised to perfection, I didn't realise how much I fall back on fingering defaults when playing from memory.
Fingering defaults act like hints when playing from memory. If you're not quite sure what comes next, well, chances are your fingers already know the pattern and will take care of it for you. That is, if the fingering required to play the passage matches those generic patterns. In Bach this isn't the case; the fingering patterns are unexpected, almost unnatural to a modern pianist. You can no longer depend on generic fingerings - they simply won't work.
Perhaps learning historic fingerings that keyboardists actually used in, say, the baroque era would enable one to develop a new set of fingering defaults that make playing music of that era much easier. This is something I'm very curious about. However, it's important to keep in mind that historical fingerings were not destined for the piano, or at least the piano as we know it today, and different fingerings are probably necessary to play early music on today's pianos - which have far heavier keys - without developing RSI.