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.)
Best Answer
Firstly for an initial point of no return - as soon as you get access to the Sirta vendor in the Hospital ward buy a/the present/s, completing missions removes them quickly before you know their value.
Secondly before beginning Priority: Tuchanka you should finish off as many sidequests as you can, since completing it renders some sidequests as failed. Don't panic about having some remaining in your journal however, a few don't have their locations unlocked for retrieving quest items until later.
Thirdly Priority: Cerberus Headquarters is the point of no return for the rest of the sidequests. You can still use the onboard vendors on the normandy to purchase goods, but because of the price markup on the ship it's better to visit them directly for whatever you need before commencing this mission as well.