I can't enumerate them all for you - I don't know them all. That's the point though - you can't know all the systems you may be breaking by changing the way your MAIL FROM works.
Here's a few that I've told you about before:
Whitelist/Blacklist entries
Quarantine systems
Anything based on .qmail files
Some auto-forwarders
Auto-responders
Now you can potentially whitelist around these issues (to some extent), but while that works for geek mail systems it doesn't scale up very well. I know you (personally) whitelist around some problem systems in your implementation but can you expect Grandma to do that?
Matt.