Firstly, I think it’s pretty obvious that the “Observer Pattern” is a very standard and very useful methodology that one must use for any number of reasons.
Added: Perhaps a better way of putting it would have been “Something to be mindful of”.
My concern, and it’s probably a small one that’s easily addressed, is that I’ve seen bugs in games and even OS’s that seem to be a result of using the idea “wait until you’ve been told that something changed before doing something” (i.e. the Observer Pattern). A possible (I’m not certain about this) example I’ve seen has to do ironically enough with cursor affordance. I’ve seen games where if one alt-tabs away, the cursor is changed by the OS to the standard pointer but when one alt-tabs back to the game, the game doesn’t change the cursor back because the conditions in the program haven’t been met to send the notification out that, for example, the layer hit has changed so the cursor needs to be changed.
Hopefully it doesn’t seem like I’m rambling here. 
