Story - very River-centric. River is 'Serenity's Willow. The backstory update for new viewers was handled pretty well.
Themes - Love (in many kinds) threads through the whole movie. Also, it continues the series' theme that the central authority is not deliberately malevolent, it (and its supporters) just believe they can make a perfect 'verse. By doing some questionable things. Oh, yeah.
Main Villain - nicely three-dimensional.
Others - Reavers!
Surprises - a couple of shocks in store.
Series plot resolution - one definite, another strongly hinted at.
Zoe - vengeance mode ON!
Inara - had her moment.
Mal - back to Captain Bleak, until he realises he has something to fight for.
Nice 'Forbidden Planet' homage.
EVERYBODY got beat up on to a lesser or greater degree. There was a point where I seriously thought they were all gonna die.
Slashiness - Mainly Mal/Simon, early on. Simon/River just squicks me so I didn't see any of that.
A real rollercoaster ride.
I had a couple of issues with the translation to the big screen & plot holes. But I'll wait until I see it again before making more comments.