Very often in a game like this, you hit certain landmarks in the plot beyond which a large portion of the content becomes locked out.
In Mass Effect 1, there was a sort of dual one when unlocking the Ilos mission (being unable to access the Citadel) and actually going to Ilos (being unable to leave).
Mass Effect 2 had a different variant on this, where if you didn't complete pretty much everything you wanted to do before the IFF mission it was impossible to do without causing some unpleasant consequences (and skipping it and heading straight to the endgame to avoid those consequences caused OTHER consequences).
Is there any point in Mass Effect 3 like either of these scenarios? Is there some task you should be sure to hold off on until you're ready, or you risk losing access to content? (I'm not looking for a laundry list of things that you can or can't do - a good answer would be a very clearly demarked "Make sure you've done everything you can or want to do before you go do Mission X", if there is such a junction in this game.)