I posted them in the OOC thread, but I'll tell them to you here to.
1: It starts at a really low level, like 5 at the highest.
2: It is the only pokemon that you start with.
3: There MUST be a good backstory behind it. (This is probably the most important one.)
4: No Gracidea flower for a very long time. Like, not until you collect six badges or more.
5: When you get those badges, you must then complete some long side story to get Gracidea flowers for Shaymin.
Can you handle all that? Not a bad trade off if you ask me. I think you can. ;)