Bach was an extremely expressive composer, but his musical expressivity has very little to do with the kind that we encounter in Beethoven, Rachmaninoff, or even Mozart for that matter. Virtually everything in Bach's output is an exercise in taking established procedural rules for music-making and then bending them to his own purposes, often while consciously obscuring from us the means through which he did it, with the added vexing caveat that the stuff has to sound good and be danceable.
Bach's music is onion-like in this idea of layers: not just the actual musical layers of polyphonic imitation inherent in the music itself, but in the aesthetic and philosophical layers of thought and consideration that went into his work. There is always a sense in which Bach is consciously trying (and, in my view, invariably succeeding) at being cleverer than we are, at achieving iconoclasm through synthesis and sleight-of-hand rather than a more turbulent or destructive course of action. I think that this comes across in his music, whether the listener is conscious of it or not, and it is quite off-putting for some listeners. There are times when it can become trying or tiring even for me, about as big a Bachophile as you're likely to encounter at large in the world.
Bach is a continuous stream of process, not a punctuated string of big moments. He never gives us exactly what we want; he is continuously pushing us further back from the goalpost: there is a sense in which his music is deeply human, but also a sense in which it refuses to join us on our own ground, and I think that's a valid, if perhaps short-sighted, criticism.
Bach is about taking modest means, in terms of materials and musical rules and procedures, and generating the greatest possible variety and scope of results from them, rather like the biological world as understood through evolutionary thinking. He is a musical MacGyver, to be honest. There is something in Bach which will tend to appeal more to those who understand music as a vocation, an interest, or an occupation (in the literal sense) than to those who value music more as catharsis, release or statement. That's not intended to be a value judgment: it's just the nature of the music in question, I think.