Hello there Duncan, wow I love this question! Thanks!!

I wrote all this response this morning, then took some time to see it with fresh eyes, by that time you'd already got the answers anyway

- still, I think this is worth posting, there is details here that might be useful to someone.
What we find as we look into this issue (the failure of Christianity to yet achieve it's full objective) is that Jesus did not fail, nor did His disciples fail, but those taking His name for vain purposes, (self-serving Christians and the world at large), has failed to repent and fully embrace Christianity to the extent that can overcome evil.
In the scriptures of The New Testament, we see evidence that within the churches were people rising to leadership and teaching positions, that were doing so for selfish reasons. In other words, their preaching was not because their heart was fully in service to God, but because they enjoyed the recognition/benefits of being esteemed as an authority in the church, and they found that they had some talent that made them particularly successful to that end. We observe this even in the modern days.
For example: 2 Peter 2:1-3, Jude 1:1-25, Acts 20:29.
Jesus taught about this too (Luke 19:14, Matthew 13:24-30).
In St. Paul's writings, he frequently addresses them "to the holy ones in the congregation" - explicitly stating that not all of the congregation can be addressed as holy ones. Even in the church today, we see that congregations contain mixtures of people holy and unholy, .. the ratios vary of course, but it is my firm commitment to say that this has observably lessened over the recent years. The cause is easily attributed to media influences - a noticeable absence of Holy Spirit in public Television and Music, has groomed social attitudes to be more inclined toward sexual immorality, more obsessed with consumerism, more interested in gadgets and fun, to the detriment of our relationships with God and the people around us. These media influences have taken the primary place in our lives, so God has been relegated to mealtime prayers and an hour on Sunday. Subsequently, there has been a gradual lessening of love, respect, compassion, ethics in society, because of our increasing tendency to self-gratification (Matthew 24:12).
So, the earth as a collective entity, still has not overcome the enemy, and the enemy is advancing upon it. The enemy is sin. The enemy is purely and deeply evil, it's heart ultimately desires destruction and death. It creeps into our lives with cunning - appearing at first to be harmless and good, gradually taking hold of us until we are outright rejecting and opposing righteousness, also rejecting and opposing The Holy Spirit of Truth.
Our enemy has strength against us because of our selfish ambitions. We all have weaknesses, that is true; we are all human, and humans have common, natural desires (comforts/pleasures, ego etc - 1 Corinthians 10:13). The particular nature of the people around us, the experiences we have, the way we learn to cope with them, the way others demonstrate survival to us - life just causes us to be more or less inclined to those desires in our own unique way. The enemy, being a personal enemy to us, observes and learns of our weaknesses so that it might have mastery over us for it's own purposes (Genesis 4:6-7).
Yet the gospel of salvation in Jesus Christ states that if we surrender our life for Jesus' sake, we will receive life abundantly; and conversely, the one who desires to keep his life for himself, will ultimately lose it. Furthermore, we are obligated - not merely asked - to pick up our own cross and follow Jesus. This is an ultimatum: will we stand up for the truth and for Jesus' vindication; or will we allow the enemy to prevail against us and to use us as an instrument of destruction in His name? The gospel is more than an evangelical sales pitch.
To be a successful ambassador for Christ, we must be righteous. We must be able to stand against all accusations without blame, so that God will be pleased to defend us (Proverbs 18:10). We don't have righteousness of our own accord, because our past has sin. Yet, because Jesus has purchased us from sin with His own blood (1 Corinthians 7:23), the burden of that past belongs to Him, and we are graciously gifted with a good conscience (2 Corinthians 5:17). But, as Hebrews 10:26-29 explains, there no longer remains any sacrifice for sins after that. We must repent perfectly for the rest of our days, in order to maintain our confidence of righteous standing. Our righteousness does not come from being able to demonstrate to our fellow man that we comply with a law, rather, we know in our own conscience that God is pleased with the way we are living (Romans 3:28-30).
Righteousness is paramount, because it is power to condemn the sinner (Matthew 5:20, Isaiah 54:17).
The true disciples who first carried the gospel, certainly understood this well (it is visible in their writings). Yet they also saw that the flesh is weak, and the most appealing influence within the congregations was of a sinful persuasion. They saw that the enemy was continuing it's assault upon The Kingdom of God. St. Paul writes in 2 Thessalonians 2 "The day of The Father will not come until the great apostasy comes first". And so, we do see signs of this in the present age: churches have compromised the standards of holiness, in order that they make it a welcoming place for sinners, who fill their seats and pay the bills.
You asked "why is it taking so long for Jesus to return?". This is it, really. God needs us to rise up and speak these truths on His behalf, but we have largely become impotent, lukewarm. Also, the lost sheep of Israel are still lost. The Kingdom of God can only be complete, and their faith can only have full power, when they accept that Jesus is Messiah. They cannot easily see Messiah in Christianity when all they can see is that Christians are slaves to sin, calling "holy", things that are not holy, and saying that God's law is not relevant (because they are lawless).
.. and briefly about the word "generation" - it most generally describes the lifespan of an offspring that comes and goes. It doesn't necessarily mean a fixed period of years. When you read Matthew 24:34, also read Mark 8:38, and Matthew 17:17. The context that Jesus is using the term "generation", seems to describe the period of earth's history after the fall and before the restoration, the period during which sin reigns (Genesis 3:15 until Revelation 21:3-4).