Command+Tab misbehaving for apps on multiple monitors
It seems that this is related to displays having separate spaces. This can be changed by unchecking System Preferences -> Mission Control -> Displays have separate Spaces. (requires logout+login)
Note that with this you'll get the old full screen with multiple monitors support (that isn't so nice usually)
Related also to other multi-monitor functionality things, see for example cmd-tab behavior on Mavericks with multiple displays.