$650 MIL or so final numbers is quite impressive for the start of a franchise: yes large budget, yes the most "well-known" Superhero, but that doesn't necessarily translate into box-office gold, especially when your film has some (a lot) of controversy, coming off a bad reboot, isn't entirely child friendly, and has immediate and large competition the following week. I would have preferred to see a stand alone sequel, but given whatever allure The Avengers had -- 1 beloved hero, two that had fine (ok) box office numbers, one with multiple failed films, and two that no one really cares for except for a nice rack, MOS 2 unless the villain(s) were played by big box office draws, likely wouldn't pull in a billion - and this is coming from a guy who has bought tons of man of steel merchandise, pushed everyone to go, and watched it 4 times now.
The "intrigue" apparently of seeing six heroes on one screen at the same time more than doubled what Iron Man 2 - the biggest box office draw of TA's heroes is what likely contributed to the inclusion of Batman, whether for good or bad, and I see pros and cons.
