This is not a complete list of course, but it should cover the most common problems.

A lot of things are pretty lax, so don't worry! But here are some pointers:

Messing up other people's Ages

You must have a unique sequence prefix. (Otherwise the Ages will break each other. It's possible for me to fix this after-the-fact if you've lost your sources files and only have the prps; let Dustin know if this has happened to you.)
You must have a unique Age filename. (Otherwise the Ages will overwrite each other. It's possible for me to fix this after-the-fact if you've lost your sources files and only have the prps; let Dustin know if this has happened to you.)
Your custom sounds must have filenames that begin with the Age's name from above. (This one is let slide a bit, because it's not actually a problem until a second Age does it with the same filename. But it still should not be done.)

Security

Python code for Ages mustn't contain "exec", "eval", "os.system" or similar statements. (It is easy to slip in undetectable malicious code this way.)
Python code for Ages mustn't make network connections. (It is very easy to slip in undetectable malicious code this way, or create spyware. Also, it would break the user's privacy, as that would keep track of everytime anyone anywhere logs into the Age.)

E.g. Relto pages could be done in principle, but there needs to be a framework so that they do not slow startup, and they are paged out by default. This hasn't been done yet, but I've got some plans for a way that does not slow startup and all fanmade additions are paged out by default, until enabled. Relto pages are now possible! But they are by design turned off until the player turns them on. Still, one mustn't mess around with other people's Ages, unless they are intended to be as such, as is the case with Ahra Pahts.