First, note that the parables listed (about drowsiness, about delay) and the tasks given (preach the Gospel to the nations) invoke no timescale. Will the Church take a decade or centuries to "fall asleep?" When the known world is fairly small (no guess that North America even exists, for example), how long will preaching to "all the nations" take? Looking back, current theologians will say, "Obviously centuries for both," but this is because centuries have come and gone. We'll see below why this isn't fair to the text.
Matt also raises other issues:
If anything, this text could be seen to deny Jesus's full divinity; if Jesus is "fully God," how could the Father know something he doesn't? But we leave that particular difficulty aside, and focus on the fact that Jesus uses the phrase "day or the hour," which refers more clearly to a very-soon-coming (imagine if Jesus had said "the year or the decade" instead) than it does to a far-off-coming; at any rate, it's hardly compelling evidence for the latter.
One final point: in 24:36, two verses after Jesus allegedly claims he will return within that generation, Jesus reminds his listeners that no one knows the day or the hour of the Son's return, "not even the angels of heaven, nor the Son, but the Father alone."
Clearly, then, it seems unlikely that Jesus Himself thought he was going to return within that generation. One can account for his statement in 24:34 by limiting the scope of "all things" to the destruction of the temple, a limiting that seems plausible in light of the disciples question in 24:3 and in light of Jesus's reference to the "Parable of the Fig Tree" in 32 and 33.
We see that the first statement simply isn't supported in the text. The "all things" in 24:34 make little sense if they refer to the "all things" in 24:3. The "all things" in chapter three are physical objects; the "all things" in 24:34 are events (physical objects don't "happen"). Read the whole passage for yourself, and see if the meaning is as plain as I think it is.
Especially the whole "timing" issue:29“Immediately after the distress of those days “ ‘the sun will be darkened,
and the moon will not give its light; the stars will fall from the sky,
and the heavenly bodies will be shaken.’[c]
30“At that time the sign of the Son of Man will appear in the sky, and all the nations of the earth will mourn. They will see the Son of Man coming on the clouds of the sky, with power and great glory. 31And he will send his angels with a loud trumpet call, and they will gather his elect from the four winds, from one end of the heavens to the other.
Here's what it boils down to: to accept that Jesus didn't "really" mean what he said, you have to accept that "near" doesn't really mean "near," that "immediately" doesn't really mean "immediately," that "right at the door" means "miles away trudging through the snow," and, most important, that Jesus's conference with his disciples was a ruse--not just in the sense that his prophecy never came true, but in the sense that he never really meant it for them.
There, fifteen minutes, done. How'd I do?