It would be entirely different. For one, there wouldn't have been Iron Man.
The reason that Disney went with Iron Man is that they owned the rights cleanly.
They would have also tried to put the X-Men and mutants into the setting and that really doesn't mesh well at all. If they could make it work, it would have been an entirely different plotline pretty much from day one.
Honestly, if Disney cleanly owned the rights to Spiderman and Hulk it probably would have been enough. Although in that case, Spiderman would have likely been one of the founding Avengers; and we probably would have got some more Hulk movies (which no one has yet done all that well).