I've been having a lot of fun bumping into 'old' NPCs and quest givers that I've helping out on my journey from 10 to 60, or 61 to 70. They add a real sense of progression and continuity to the world. I solved their problems years ago, and that's allowed them to move on in life, for better or for worse. It's also strangely heartwarming to see people I did quests for so long ago, like a little slice of nostalgia.
I've heard a few people hating on how many old world NPCs you run into in Northrend, complaining that Blizzard is just recycling old content. I quite seriously disagree in this case. Seeing NPCs progress in the world alongside us gives a much greater feeling of the world being a story, and as we go up in level, the world moves forward. I like it a lot, and I hope it continues into the next expansion, too.
We have an old gallery sitting around of some of the returning faces you'll see in Northrend, so you can check it out if you want. Be warned, there are some minor spoilers inside.