All the seeds were there from the very beginning, but to be fair, they let each individual hero have breathing room to develop and allow us to care about them before throwing them in the stew together.
1. Nick Fury’s “Avengers Initiative”
2. Tony approaching Ross at the bar
3. Black Widow in IM2 and the post-credits spotting of the hammer
4. Hawkeye’s cameo in THOR and Loki seeing the cube next to Selvig in the post-credits
5. Essentially a preview teaser for the Avengers
Then in Phase II, they *appeared* to start new individual stories again, but…
7. Tony had PTSD from the previous film and Banner appeared in the post-credits
8. Aether introduced and giving it to the Collector at the end
9. Struecker displaying the “twins”
And then in the TENTH film, in a seemingly unrelated film from the rest, we get a whole scene with the Collector and the exposition of the Stones… AT LAST. So, even though the hints were there, I can also completely understand why some may not have fully realized it until GOTG.
Doesn’t matter how you got here. All that matters now is that you ARE here. 👍😁