Spider-Man is such a strong character on his own, and he does not need to be in the Marvel Cinematic Universe. I'm not saying it wouldn't be cool to see him fighting alongside Iron Man, Captain America, Thor, and so on. But its unnecessary. That's why I would be satisfied with just an easter egg, such as Stark Tower in TASM2 or Oscorp tower in The Avengers.
Characters like Captain America, Thor, Hawkeye, Black Widow, the Hulk, and so on don't work as well on their own IMO. They NEED to be part of a cinematic universe in order to work.
X-Men also don't need to be part of the MCU. They have a HUGE world of their own to work with. Though it would be cool to see them crossover with the Fantastic Four (which Fox plans on doing at some point).