Surely, there is often more trickyness in one page here than in many of the large pieces, which tend to be a bit more linear. It really is the most amazing collection, fully on a par with the Well-Tempered Clavier, as I feel it.
Yes, but there is an important difference to most other works, including WTC: Bach really reinforced here the (German) song lyrics down in music. In German this is called "Klangrede", don't know how to translate that. Even though these Orgelbüchlein pieces sound great without reference to the lyrics the connection is VERY strong to the lyrics. This aspect is maybe underestimated because of translation problems from German to English or other languages.
Even although very off topic, one funny thing what shows how many other aspects Bach put in his works:
In the St. Matthew passion there are passages with words about the crucifixion. One has to know that in German, "Crucifix" has a double meaning as "flat" in the music. Bach not only put lots of flats in the music while the words about crucifix appear, he also used notes for that which could be interpreted as the 4 poles of a Crucifix.
And that is combined with great timeless music. That is the real genius - to combine all that to a masterwork. Sorry for beeing off topic, but I sing in a choir the St. Matthew passion on Eastern and looking forward that great event already...