Seems fairly obvious that they are setting up a McGarrett/Quinn romance, perhaps to fill in the gaps during the 50% of episodes when Danno is absent. (Notice she always rides along with him now, like Danno has always done.)

Quinn keeps using the "all up in my business" phrase repeatedly during the few episodes we've seen, but none of the characters seem to notice. Is this going to be a catch phrase or something?

I'm actually kind of glad they killed off Cullen (provided he was really killed). He went from "guy who avenged the murder of his wife and daughter" to Five-Zero super-villain without any kind of real explanation. I had a hard time hating him.

Tani was the one who chased down the female villain at the end, so they could have that underwater fight. They never allow male good guys to fight female villains in this show, unless it's near the beginning where the female gets the better of them and gets away. When the female villain has her back turned and is running down the dock, it doesn't make sense why Tani doesn't just shoot her dead, given that that she had just shot at Tani seconds ago. Police protocol allows them to shoot criminals dead during a pursuit if those criminals have fired upon them and are still holding their weapons, even if their backs are turned. This occurred in Las Vegas earlier this year, where they shot dead a robber at the Bellagio after he shot a cop wearing a bullet proof vest, then tried to flee.