MNF games are not flexed; every team plays once on Monday (there are two games week 1 to make up for no games week 17).
TNF games are not flexed; every team plays at least once on a Thursday (though due to Thanksgiving games, it's not guaranteed every team plays on TNF itself).
SNF games are flexed starting in week 11; some teams play multiple games, some teams play none, depending on projected ratings and importance. Only Sun afternoon games can be flexed to SNF. Edit: Fox and CBS each get to "lock in" one game each that can't be flexed, I believe.
I wasn't paying attention to the TV business at the time, but it seemed like the NFL wasn't happy with the move of MNF to ESPN. They lost their spot in the Super Bowl rotation, they had to air all the games in a local market, and as the scheduling suggests, the NFL isn't holding the marquee games for them. Not that Disney cares in the least - at least $5 of every cable/satellite subscriber's bill goes to the ESPN networks. That's around half a billion a yearBEFOREyou add one commercial. Almost all estimates suggest ESPN is the most profitable TV entity by far.