Just watched the film last night, and I have to say I thought it was amazing. Really, really good. Anyway, some thoughts:
I don't think it was glass breaking that you hear when Robert stepped into the puddle. I think it was just an audio clue to let the audience know something was about to go wrong. I mean, why would he have left an intact vial of blood in the puddle? Those things are sealed, and wouldn't have lured in any vampires. Particularly since it was obscured by the puddle, too. Remember, Robert was hanging there from before midday to just before sunset. That's a good few hours for blood to drip out of his head (particularly since he's hanging upside down) and spread out in the water. It would probably appear darker due to the fading light, as well.
I thought I saw Fred's head move, too. Although I don't know if that's because I read it hear first and was expecting it
Seriously, Sam dying was way too sad.
I thought that one vampire was the "alpha male", too. At the end, in the lab before Robert blows them all up, the other vampires stand off and let him tackle the screen by himself. Even if he was doing it just to get at his "girlfriend", they certainly afforded him the respect of a leader (albeit an animalistic one).
That may have been a little rambling. Just some thoughts I had on the film in general, as well as on some of the comments here.