Kevin Smith does a podcast called Fatman on Batman. The last episode had Neal Adams on it and it was fascinating to listen to him. Adams touched on the Marvel movies and got in depth on why Marvel doesn't own the movie rights to basically any of their major characters and why it would cost a mint to get them back. Stan Lee essentially made several bad deals (the worst one was for the F4 rights) and as long as the movies keep getting made, the movie studios keep the rights. That's why there are so many B (Dr Strange), C (Antman), and D (GotG) level characters getting movies under the Marvel Studios banner.
Honestly, I think it was for the best. Marvel desperately needed money at the time, which is why they sold off the rights.
Who knows how it would have turned out if Marvel had all the rights still? If they were able to put out Spiderman, X-men, and F4 movies, we may not have gotten Iron Man or Thor. Cap it probably still a movie, but the rest of the Avengers? I don't know. I think it's very unlikely we get an Avengers movie that was as amazing as it was if Marvel has all of the rights. Iron Man is considered an A List superhero now, but that is only because the first movie was so incredible. I personally skipped the first Iron Man in theaters because I barely knew anything about him.
We definitely don't get Ant Man, Dr Strange, or Guardians. I'm really looking forward to these movies, so I think that would be a huge loss.
Now? I wish they could get the rights back (I care less about X-men because DOFP was awesome, and I'm interested to see if FOX can turn that franchise around), because I think they've found their groove and see the value in making movies about lesser known heroes. But I think the way the rights were handled led us to one of the best movie franchises of all time.