One of the principal things one notices about many 'great' works is their extremism, at least in one respect or another. For instance, this is one of the features that distinguishes Beethoven's late string quartets from his earlier ones, and indeed, what separates those pieces from many others throughout history. This extremism does not necessarily result in 'modernity', or even originality, although it can. Beethoven's late quartets are indeed often seen as being rather 'difficult' music, although much of this difficulty has disappeared to some modern listeners.
Bach's extremism took another direction - he was an extreme conservative musically, and his extremism is apparent in his counterpoint. Long, melismatic, tortuous and highly baroque lines that were old hat in comparison to the light fluidity of early classicism. Bach must have appeared to be a real stick-in-the-mud, rather than as the 'architectural' genius he is seen as now.