1) Slate. They will be breaking the news embargo on exit polls, and releasing data throughout the day. Why is this a big deal? Florida 2000, of course! After polls closed in the peninsula (Eastern time), but before they closed in the panhandle (Central time), the networks tried to call Florida based on exit polls, and some speculated that this may have depressed turnout in the panhandle. That's when the news organizations really cracked down on the exit poll embargo. This year, Slate is just going to do data dumps throughout the day, in real time. Ethical? Who cares? I'll be checking them obsessively.
2) New Hampshire. If Clinton wins New Hampshire, so much for Trump's chances, and their polls close early. So, even if Slate doesn't screw everything up for everyone, we can still tell pretty early if Clinton locks this thing up, even without the usual key states of Florida and Ohio.
3) The Senate. I haven't been bothering to talk about it, because, well, Trump, but if Clinton wins with a Republican Senate, Supreme Court chaos.