People won't ask why Cap or Thor doesn't show up in an Iron Man movie. Simply because it's an "Iron Man" movie. There is Iron Man in the title not Iron Man and his Avenging Friends. The GA isn't THAT dumb. This is just an issue for fans underestimating the GA
Yeah, I agree. It's something that will take care of itself for the most part (with perhaps a little dialogue to help it along).
To me, a bigger question is: how much "crossover" material do you want to have?
On the one hand, you don't want every film to feel like a mash-up. On the other hand, there's not much point in having a shared universe if you aren't going to have any "sharing."
I guess that's where characters like Coulson and Maria Hill come in, potentially. So, it will be interesting to see how they handle that.