None. If the unit accepts and charges off some USB port then all is good as USB on the PC is supported. So an USB charger is fine. HOWEVER all makers will void your warranty if they feel it was dinged by a non-standard (read... their own) charger or cable.
This means most things work just fine.
Your gear, your choice.
is it safe to use any other (non-original) charger or USB cable with my S3?
I have plenty of them from other devices (my old Nokia phone, Kindle), but I do not want to damage my new phone or battery...
After a quick try, both cables (of devices mentioned above) do start the charging process on the S3, but I did not try to leave it charging for longer than a second...
If, generally, you'd recommend to NOT use these chargers/data cables - what would you recommend using, besides originals?