@AWohlfarth,
Jesus did give long sermons. The Gospel of Thomas is a sayings Gospel - the idea is to take his sermons and note the sound bites, the meat of the message. Also, Thomas takes parables and gives them in brief rather than building a great deal of narrative around the parable.
The difference between Thomas and the Synoptic Gospels is mostly style. Sure, the religion in Thomas is a little more... mystical, perhaps, but not by much. The Synoptic Gospels were narratives, Thomas was a collection of sayings. It's style.
John, however, is quite different. John was written long after the Synoptic Gospels and long after Thomas. It is also likely that John was written, in part, in response to the Gospel of Thomas given the theological disagreements between the two and the remarkably negative portrayal of Thomas in John's Gospel.