My girlfriend and I are going to see the new movie as soon as we can after it comes out; we were hoping for the 18th but I suspect that everything is booked with preorders by now.
Anyway, predictions:
Luke Skywalker is going to die, probably via Kylo Ren's lightsaber. Luke is the Old Master now, just like Obi-Wan Kenobi and Qui-Gon Jinn, and both of them were dispatched by their trilogy's badass-looking Sith Lord in the first episode. Also, Finn, Rey, or whoever else is Luke's apprentice will scream "NOOO!" upon seeing this.
Alternately, Han Solo could die. Harrison Ford has been wanting to kill him off since Episode V. Either way, I fully expect at least one member of the original cast won't make it out alive.
Somebody's hand/arm will get chopped off by a lightsaber. I don't even think I need to justify this one.
Since it's been confirmed that Admiral Ackbar is going to be in the movie and JJ Abrams loves callbacks, the line "IT'S A TRAP!" will be uttered at some point.
Kylo Ren and Rey are part of the Skywalker family. The former has some strong ties to Darth Vader, and the latter is a Force user with some kind of family she's been waiting for. If Abrams does want to use some Expanded Universe elements (which I think he was open to), I wouldn't be surprised if they turn out to be Jacen and Jaina Solo. Alternately, Ren being Luke's son could make sense too.
The mask Kylo Ren wears is hiding something important. The only time we see him without it is in the duel against Finn, and that's only from behind. So the question is: Horrible scars of some kind? Family resemblance to a trusted mentor (ties in with the above)?
Kylo Ren may not be as loyal a member of the First Order as assumed. First, there's the usual caveat of the Rule of Two and he's almost certainly looking for ways to kill of Supreme Leader Snoke. Second, his statement "I will finish what you (Darth Vader) started" was suggested once to be incomplete, with the complete version being "I will finish what you started when you killed the Emperor." It was always Vader's plan to unseat the Emperor and take the Empire for himself; Ren is most likely trying to do the same.