There were two little moments in Serenity that stood out as my favorites. They were love letters to the fans snuck in completely undercover. Has anyone posted about these yet? My apologies if I'm mentioning old news - just saw it last night for the first time and tried to remain fairly spoiler-free till then.
A little ways into the movie, Jayne and Kaylee are in the cargo bay, and Jayne is, well, it looks like he's getting ready to toss various debris out. She's talking to him and fiddling with a part. It's the same part that broke in "Out of Gas."
There's a short scene where Mal isn't wearing a shirt. Is that a spoiler? Anyway, he has a little scar on his chest. In the same place where Crow hit him with a knife (and Simon later stitched up) in "The Train Job."
I love that those moments were there. They were winks to the fans, but subtle enough that someone who came into the movie cold didn't feel left out of the joke.