The main thing that you cannot do of course in discs 2 and 2 though is naturally you lose Aeris from the party, due to, um, a certain event that really doesn’t need any further discussion on 😉 Only by using a cheat device or a save file trainer (PC version) can you continue to have her in your party after that point.
There’s an FAQ on the subject at www.gamefaqs.com if you’re interested.
The main members of the development team, in particular, the writers (namely, Yoshinori Kitase and Tetsuya Nomura) have stated in the past the reasons for Aeris’s demise, and that it was always planned from the beginning that she was meant to die as part of the storyline, that it was to destroy the Western cliche of a ‘noble sacrifice’, by instead depicting a narrative where a main character dies suddenly and unexpectedly.
So anything else mentioned to the contrary is incorrect.
Now that I think about it, I’m fairly sure the whole "It’s a game that teaches you that when someone is gone, as long as you remember them they’re not truly gone." vibe came from the fact Hironobu Sakaguchi had lost a dear relative at the time.
Though i’m confident that one came from the man himself. I should remember where I read things really :P.
During the development of Final Fantasy VII, Hironobu Sakaguchi’s mother died. At the time, Sakaguchi wanted to craft a story that told of how just because someone has passed away, doesn’t mean in any way that they are gone, and to show a realistic death rather than a "Hollywood" sacrificial death that previous games in the series had done. These desires developed into the Lifestream, and Aeris’ iconic death scene and subsequent continuing role in the lives of the cast.