Co-op matching with friends is impossible. The game freezes on the Lineup screen 90% of time.
Not sure why sending a invite to a friend is not an option either.
While playing I noticed a lot of outfielder errors, but this could be the players I've played with too.
Also while baserunning on classic, the right stick will make you run back to the previous base instead of allowing you to slide in the direction you want.
Co-op seems like a really great idea. Having a league, or some way to track stats for the players would be really great. Setting up tournaments would be a really cool way to reward players for playing the mode.
Also not sure about allowing people to create custom teams to play with. This would be similar to DD or maybe even using the users teams in some way in a special mode of Co-op.
Overall just suggestions thought of and errors I ran into while testing.