Mass Effect 2

OK guys, finally playing ME2 for the first time and I need to know something: until what point of the game can I mess around the system?
I know that I have to do some shit before the Omega-4 Relay then I can't go back, are there any other critical points that have some optional (not noted by the game) requirements or limit the completion of unfinished side missions?

A mission to retrieve an IFF essentially puts a timer on your time to do side missions before the Omega-4 relay, if you are going for full completion, complete your side missions before that one.