I suppose my "holdup" so to speak, is that I have yet to see how they fit. Assuming Joss does what we all hope he can do (keep the awesome of Avengers flowing into the sequel) then there won't be a problem.
But from what I've seen, the MCU has done well because of how much they explain and how successfully they explain it. Each film builds on one another in a way that makes it all seem plausible. Coincidentally this is likely why Nolan has been succeeding with his "DCCU."
Now, I am a big advocate of Dr. Strange's addition to the MCU, A. because I love the character and I think he'd look amazing on screen. and B. because the way his story is told in the comics makes magic seem reasonable and to put it simply, rare. He is not born with the ability to do magic. No one is. But thats just it, I want Marvel to give Strange hours and hours to explain why magic works and fits in the universe. And I want them to show that he is one of the very, very, very few who can, especially on Earth. And only because of how hard he trained and how strong his will is.
I think part of the reason I fell in love with the MCU is because mutants aren't allowed. Each hero is unique and their power is immeasurably astonishing because no other is like them. Their origins were intensively and meticulously integral to their characters. The addition of two characters born with their powers (who may or may not require any origin at all) is just... bleh to me.