My emotions are mixed right now. I've been wanting this forever, and never thought it would ever happen. At first, ecstatic, with tears in the eyes. Now, all I can do is worry that it won't come close to the original.
Reading through this thread you realize just how many special moments there were in the original game that will be very challenging to remaster. This is a huge undertaking by Square.. HUGE. Will they treat it with the respect and delicate hands that it deserves considering it's a 'REMAKE' and not a new game? Sometimes when you're working on old material the motivation isn't what it is if it's all fresh and new.
I think it comes down to how special this game really is to those guys. If it's as special (or moreso) to them as it is to us, then I could see this being, hands down, the greatest game ever made -- just as the original was. If it's not.. if it's treated as something that just 'has to be done to appease the masses' .. then it will be soulless and a huge letdown.
That all said, here are a few 'must haves' for me.
1) World map - I don't care how they do it, as long as they bring some sort of world map back that you actually explore. None of this dotted line-travel to new area crap like in Lost Odyssey or some other recent RPGs. Being able to travel the world map makes you feel so much a part of the world.
2) Soundtrack - Do not bastardize it. We don't need a techno/hard rock/anime remake of everything; just give it more modern VIs so that it fits in this decade, but keep the original melodies intact.
3) Story - Keep the core intact. I think all of us are OK with them introducing a few new elements here or there (who wouldn't love new missions or side quests that give more insight into the wonderful world they live in?).. but don't change what was already perfect at its heart. If they do this, I think it will be a huge mistake.
4) Secrets - The original game was filled with all kinds of secrets. Materia, chocobo farming, weapons, etc. Keep the discovery element alive by having the same kinds of things in the remake (changing the location/timing of these would be fine, but keep them involved in some form.)
5) Aeris - While most of us cried when Sephiroth took her away, don't go changing what is still to this day the most emotional moment ever in a videogame. I wouldn't mind if they introduced some way for you to interact with her later on in the game (whether it be something spiritual or something along those lines) but, again, don't mess with the core.
6) Story Milestones - Building on the Aeris mention, keep, and even enhance, on the milestones along your journey. Aeris' death, Seto's death, Cloud's past, etc. These were all emotionally charged moments that kept you playing the original all through the night without bathroom or food breaks. Don't toss these away for the sake of 'something new'. They were perfect in their original form; just make them better.
I could go on for weeks, but those are some at the top of my list.