I consider a replay to be great when the maker thinks out of the box and actually try to do something no one else is capable of doing. No matter how absurd the idea is, the replaymaker would move heaven and earth to bring his idea to perfection and claim the replay as a huge accomplishment.
A lot of people don't understand that we need this kind of mindset to actually be outstanding.
Dramat-ism aside, I determine great replays by imagining the effort and time put into it and knowing that no one else can possibly do better.