Conzelmann sees Lk 16:16 (unique to Luke) as the key passage representing Luke's view of John the Baptist. John belongs with the prophets as the greatest of them, but he is not a part of the time of Jesus, when the kingdom of God is proclaimed. John is no "forerunner" of Jesus, but only a part of the age of the prophets that point to him. Jesus is an eschatological figure, John is not (101). The main points of redaction that indicate this are (1) Luke's omission of "after me" from Mk 1:7, and (2) Luke's removal of John from the scene where Jesus is baptized. This puts an actual break between the two.
It should be noted that Mark doesn't have John the Baptist mention the Kingdom of God either (contrast Matthew, where John and Jesus say the exact same thing), which means that Luke could just be using his sources. Still, Lk 16:16 does appear to make a pretty clear claim, and these two points of redaction in chapter 3 appear to substantiate it. (A weakness of Conzelmann’s emphasis on 16:16 is that Luke doesn't draw any attention to it; it’s sort of thrown into the middle of a chapter on money, along with a teaching about divorce.)
Conzelmann claims that "John does not declare that judgement is near, but that the Messiah is near" (102) –– i.e., that the judgement at the Parousia remains far in the future. I'm not sure I buy that, since the axe is still at the root of the tree in John's proclamation.
So then, Jesus is the first to preach the Kingdom of God (114), but that kingdom will only actually come at the Parousia (107). In the meantime, we can see the kingdom (123) by looking at the life of Jesus, but it is not yet present.
The actual proclamation of the kingdom in Luke has a different purpose than it did in Mark. In both books, the arrival of the kingdom is an eschatological event. In Mark, the kingdom is proclaimed as arriving quickly, because Jesus actually inaugurates the end times; in Luke, the arrival of the kingdom is placed far in the future, so that the age of the church can be seen as sustainable indefinitely. Luke, then, places an emphasis not on the proximity of the kingdom but on its nature (104). Cf. Lk 21:9: "The end will not come immediately."
The main problem with this claim is Lk 17:21: "For, in fact, the kingdom of God is among you." Conzelmann claims to explain how this fits with Luke's eschatology (120-125), but in fact he never really does. Instead he just says something like, "Yeah, it sounds like Luke is saying the kingdom is already present; but we know (from, e.g., 19:11) that the Kingdom is only in the future, so Luke isn't actually saying the kingdom is already present." In other words, the verse doesn't fit his theory, so he pretty much ignores it––while masking the fact that he's ignoring it by pretending to discuss it.
Conzelmann basically says (123) that 17:21 explains only the nature of the Kingdom, not its time. But if the arrival of the kingdom will mark the very end, in what sense could it possibly be "among us"? Won't it be externally obvious once it comes, thereby making Jesus' statement meaningless?
Conzelmann has an excellent summary on pages 131-132.
Conzelmann emphasizes (129) that Luke takes materials which were eschatological in Mark and fits them into future events from the perspective of the church; clearly this idea of the delayed Parousia is important in Lucan studies. But considering that we can see the effects of the delay even within Paul, I can't help thinking that it was not simply a linear development from Jesus, through Mark, and into Matthew and Luke. My contention would be that it's more likely that the churchs of, say, the early 60's tended to see the end as far in the future. Mark, as the siege on Jerusalem became common knowledge, told his story of Jesus with the inclusion of a lot of apocalyptic material because it seemed the end was coming. Perhaps Luke, somewhat embarrassed by attitudes such as Mark's that tried to rush the end, wrote his Luke-Acts in such a way that the Parousia was again far in the future. My point is that Luke may have reacted to Mark, but that doesn't necessarily mean that the development of Luke's position was linear.