His handwriting is super clean and careful (compared to some other copyists of his time). Of course, he frequently uses soprano, alto, and tenor clefs but they are nothing more than one more addition to our arsenal of two clefs that we as keyboardists commonly use today. Additionally, pieces which don't require the use of two manuals are written on two staves only as was customary at the time (and some compositions are written in open score notation with four staves - as BWV 1080).
As I was preparing to play the recital of Bach's works which will be tomorrow at my church (on the occasion of his 330th birthday), I was researching the internet archives of Bach's scores. It turns out that IMSLP has quite a few numbers of his works in original notation (either as autographs, 1st editions, or contemporary copies). If you are reading this post as an email, click here to see the excerpts of these original sources.
On the same note, I'd like to point out that reading various clefs is very beneficial to transposition - in order to transpose by any specific interval up or down you just have to change clefs and the notes will stay on the same lines. Transposition in turn helps very much in improvisation because one of the most common trick here is to transpose (and modify) a musical theme throughout the course of improvisation.
So in a way, when we read music in original notation, we can have a glimpse to the mind of Bach. Basically, we can have a feeling of what it took to be a student of Bach at his time.
And now we have the chance to do the same.