Except the Starkiller base was mentioned very early in the movie. Hux mentions something about a weapon being ready. Can't remember which scene though, I'd have to watch it for a third time. But it is mentioned a few times before it is actually shown. It's hard to catch though because Hux is hardly the most interesting character and whenever he spoke I just sort of replaced it with "blah blah First Order, blah blah, New Republic"
If you're weary of super weapons, that's fine, but I guess I wouldn't say they pulled it out of their asses in the third act. Is it a bit abrupt? Maybe. Is the presence of a super weapon established though? Yes.
I would say in terms of plot though, destroying the Starkiller base was not the main focus: saving Rey was. The Resistance flying around in their X-wings trying to blow up a thing happened mostly in the background while Finn, Rey, Han and Chewie were doing other things, and the focus remained on them. The Resistance probably only got less than a minute of screentime at a time (I'm talking when attacking the Starkiller base).
Anyways....
Just watched it for the second time and noticed a few things.
1) I don't think there's a thing with Chewie and Leia. They hug the first time they see each other, and later when Chewie supposedly walks away, it looks more like he is following Finn, as they take him to a med bay or something.
2) Originally, I thought Rey acquiring Anakin/Luke's light saber and suddenly gaining wicked lightsaber skills was not great. After second viewing, she isn't really fighting Kylo Ren all that well, until she gets her "Force Nirvana" moment and lands a few skilled blows on him. This part is settled for me now.
3) I noticed this the first time, but Carrie Fisher's daughter Billie Lourd was one of The Resistance officers. Fun stuff.