Since this chorale prelude needs to be performed in a very slow tempo, one of the most commons mistakes I see people make is the lost feeling of the pulse. In such a slow tempo, the danger of playing without a steady pulse is great.
The thing is that the highly ornamented melody in the right hand part create many rhythmical problems for some people. In order to avoid the rhythmical confusion, they try counting every eighth note.
But no matter how hard are they concentrating on playing the rhythms right, they have to be aware of the entire metrical structure of each measure. In other words, they have to constantly know which part of the measure they are in.
One easy way to achieve this is of course count out loud the beats (one, two, three, and four). Moreover, you have to make the beats two and four shorter which will make them sound softer. In this manner you will achieve the alternation of strong and weak beats, and consequently – the feeling of the flowing pulse.
Another common mistake is in performing ornaments. Some people still play the mordents and the trills starting before the beat which is not correct. No matter how many notes the ornament has (3, 4, 6 or even more) the first note of the ornament in this style has to start with the beat.
Since the left hand involves playing two parts (alto and tenor), it is difficult for many people to articulate them properly. What happens is that in the right hand they try to play using articulate touch but in the left hand very often the notes sound legato.
This is partly because their left hand technique is weaker than that of the right hand and partly because their fingering might not be stylistically correct. The ideal fingering for such piece should help to achieve the correct articulation naturally almost without thinking.
By the way, check out my brand new BWV 731 Home Study Course in which I teach this chorale prelude with live sound examples and even provide a practice score with complete fingering and pedaling written in for the most efficient and stylistically correct performance:
BWV 731 Home Study Course
To your success in Bach organ playing,