I don't know if I'm in the minority here, but I think TASM1 definitely was and had a self-contained, standalone story. Just because there are other, smaller plot lines and undertones that are left to be resolved in later installments, doesn't necessarily mean the movie isn't complete. I personally like how there's (from what it seems) going to be strong ties from one movie to the next, instead of completely standalone independent movies with borderline unrelated plots. That's just my opinion tho, not griping or praising either way
and before anyone jumps me ha, even though it seems like basically the center of the trilogy (which it very well may be) I consider Peter's parents' "mystery" a 'smaller' plot line because this story was, is, and should be about Peter and him becoming a hero