First, a thought about those box office tweets. Is that a phenomenon specific to FF, or is the same thing happening to all movies lately? You have the folks who are going opening weekend to most things, no matter what, and everyone else is in "maybe I'll get around to it" mode?
Now on to the movie. I enjoyed it, a lot. I can see why some might think it is boring. For the most part, it lacks action. The Galactus meet up + escape and the climax are it. Everything else is a movie about this small, close nit family. I think that's why Galactus doesn't even really need to be all that interesting a villain. It's not about him.
Now, that sort of heightens the point about the length and pieces missing to the characters. Ben has only a little moment struggling with his permanent rock state. He has no opportunity to really explore a relationship the woman he met on the street. Same for Johnny. There is an obvious feeling of being ignored and overlooked, but it doesn't get a pivotal moment. I guess the decision was to focus on Reed and Sue and the little conflict about how he considers the terrible choice and she knows it. Even that isn't fully fleshed out.
I'll be honest, those thoughts were lying under the surface for me during the movie. I think that's because, again, it's a very emotional, family centric story. As a parent of young children, the stuff centered around the idea of anyone being ok with handing a baby over to a cosmic monster was a little tough and had me completely invested. But I'm a huge sap.
Finally, I will say it was a little amusing that they just hopped on the ship and flew off...did Galactus forget about his tractor beam? (Or was that the point of Johnny blasting all those big cables? Maybe I missed some dialogue about it)
I thought it was an awesome movie