Before I begin, I'm going to just go ahead and assume you have a Bachelor's degree in film studies with an emphasis in screenwriting. Moving on...
For starters, the movie has no tension and therefore no big pay off. What are we to expect? That Joy and Sadness get lost and don't make it back to "head"quarters? Would never happen. In that case, the second they get "lost", its not a question of "will that make it back?" but "how will they get back", and for me at least, that doesn't make for much of a compelling story.
Also, the movie is flawed from its outset. You essentially have only 5 emotions that run a person (not considering a myriad of other essential human emotions), but 4 of those are negative vs 1 positive. Are you trying to inherently say that people are emotionally overruled by negativity? Joy is annoying because she is the only positive person in the brain. You know those "happy for no obvious reason" people that you just want to punch in the face sometimes? That's your main character! And while she does go through a bit of a change through the movie, its pretty strange to have characters that ARE emotions...having the capacity to possess other emotions...Maybe that works for you, but it doesn't work for me because it directly contradicts their very character essence.
Bing Bong stealing memories too is something I can't get past. Why does he start off as such a shady character who might be up to something...only to then save them at the end?
Trust me, I "get" the movie. I get that it wants to tell kids its possible to look back on fond memories with a twist of sadness because you remember the good, but you're sad you can't go back and you have to move on, leave behind imaginary friends, etc. It's all heart-meltingly Pixar-y and ::sigh::....just could have been SO much better.
Its sad when the best part of your movie is the side gags during the end credits.
Look, I get this movie is for kids and therefore shouldn't be as highly scrutinized in some people's minds. But I don't buy the kid's movie "free pass" mentality. A flawed movie is a flawed movie. Are you going to tell me that every movie made "for kids" is not able to be criticized?