Dipper's Guide to the Unexplained
Dipper's name
In Fight Fighters, Dipper types in three letters to "save" his "game". These three letters are DIP. Normally, when someone does this, they type in their initials. Sure, it could just be an abbreviation for Dipper, but I think it's his initials. That would make his name Dipper I. Pines. This would also mean that Mr. Pines's first name begins with an i.
In Double Dipper, Wendy says "So that's how you got your nickname!" (referring to Dipper's birthmark) Some people take this to mean that Dipper is not his real name. I originally thought that's ridiculous. The other clues just didn't seem to add up; there was no other evidence that his name is not Dipper. That is, until I saw the clip from the LA film fest. In an alternate scene from Dreamscaperers, Bill says that "Your real name isn't Dipper." Because they could've easily included this detail in the final cut, I regard it as canon.
In DGTTU - Stan's Tattoo, Stan writes the word "Goober" on Dipper's face. Goober means peanut in the American South. Dipper also dresses up as peanut butter on Summerween. This is NOT evidence that Dipper's real name is Goober. Apparently, Goober is also a lighthearted insult. Basically, it's a Disney-friendly way of saying "idiot" or "moron".
In MGT Art, she shows a picture of "Dipr" when they were 5. This was my evidence before the LA film fest.
So what is Dipper's real name? Well, think about it: if you were Dipper, and you had another name, why would you not just go by that name? This is especially evident when you realize that kids made fun of Dipper's birthmark before he started hiding it. I think that, if Dipper has another name, it's even more embarrassing than the one he uses today.
other
Dipper obviously didn't know anything about the secrets of Gravity Falls up until he found the journal. I'm led to believe that Gideon also stumbled upon his journal, as he doesn't seem to know too much about it either. (He didn't even know that there was a third journal.)
There's a popular ship of Dipper and Pacifica. For people who do ship that couple, I would recommend re-watching The Golf War, since it contains more direct interaction between them than all the other episodes combined. The last scene is particularly amusing. When Pacifica is in the car, headed home, Dipper is looking the other way with his arms crossed, as if he is trying too hard to hate Pacifica. Interpret this how you want, but I think it's about her rivalry with Mabel, rather than any feelings he has for her. I am completely neutral on the subject, and I think the shippers tried way too hard to find something in Northwest Mansion Mystery.
Now, there is a constant theme, especially in the second season, of people who are "in" on the Gravity Falls conspiracies trying to keep Dipper out. They've all got big plans, and they don't want him getting in the way. I heard a theory once about Dipper being the "chosen one" because his birthmark is the Big Dipper. There was some merit to the theory. I feel like the conspiracy would have resulted in some huge mess, but Dipper will come and clean it up.
adult image
Alex Hirsch said it himself: Dipper wants to grow up too fast. This is especially showcased in The Inconveniencing, when Dipper tries to project an adult image to Wendy and her friends, and in Dipper vs Manliness, when his entire subplot deals with his manliness.
This comes up again in Fight Fighters. Stan challenges Dipper to either go face Robbie, like a man, or hide indoors like a wimp. Dipper had nothing to prove by facing Robbie. By the same token, he had every reason to face Rumble, to own up to what he had done.
