How do we hold together the Bible's call to be content in all circumstances with the fact that the Bible itself points us towards a glorious future hope for which we are to be yearning and looking forward? How can we be content in the here and now, whilst acknowledging that we don't now have the one thing which we truly need and (at least sometimes) want - the presence of the Lord Jesus? How can we be content in a broken world that is full of both suffering and evil (not to mention our broken selves, which are similarly stuffed full of woe and wickedness), whilst seriously and genuinely waiting for the redemption of creation?
The answer must have something to do with the relationship between the first and second coming of Christ. One way of thinking about this relationship might go something like this: Jesus came to get the ball rolling on salvation, and he will come back to put the finishing touches to it; in the meantime he is, through the church, carrying on the plan. On this scheme, I can understand the discontent that we are meant to feel - it's the discontent of a half-done job - but not the radical contentment to which we are called.
I think the Biblical relationship is more like this: Jesus came to accomplish salvation; there is nothing left to do, and the point of the continuation of history is just to give space for people to come to acknowledge his salvation, enjoy it, and bear witness to it. His second coming will be to reveal that salvation as it has already been accomplished, thus rolling back the darkness of sin and suffering which still clouds our view of his victory, and vindicating both himself and all those who have trusted in him. Contentment, then, is based on the accomplished work of Christ - everything needful has been one. Yearning is based on the hidden nature of this accomplishment - we want to see him glorified!
And if that's right, then one aspect of advent must be mission. We want to see him in glory, acknowledged and worshipped for all he has done. That will happen, and we can rest in the knowledge that it will happen. But a part of our waiting will surely be to bear witness in the darkness to his great light, so that we already see him glorified in the lives of people who come to know him.