What is Error 73 in Diablo 3? You usually get error 73 (even error 74 sometimes) after the servers are down for any reason and come back online in the evening. Why we get error 73 in Diablo 3? Because the login servers have a limited capacity and if millions of players attempt to get into the game in the same time, the servers just can't handle that kind of load. This can get very frustrating, but we really can't do anything, except hoping that Blizzard servers won't go offline in the rush hours until they will fix this login problem. Error 73/74 usually follows the 'almighty' error 37 (which you get when the server is busy).
When you get Error 73 or Error 74 in Diablo 3 it means that the login server is online and it is trying to authenticate you, but it fails because it must authenticate thousands of players every second, because everyone is trying to connect in the same time frame. I think this is the logical explanation for this frustrating error in diablo 3, named Error 73!
Lets hope Blizzard will fix these login problems, because if this will persist and if for some reason they will need to get the servers offline in the evening, people will get very frustrated when the servers will come back up. But as they announced earlier, they are working very hard to resolve these issues with the Battle.net service.