From a user interface standpoint I think centralized authentication is one of the most important things.
Consider an intranet. If you have one password to log into all of your enterprise applications you are going to know how to log in and you will use all those applications.
If you have 20 different applications you use at work and you have 20 different logins, probably some of those services will end up in a state where you have to bug IT people to get a password reset when you ever need to use them. There will be others you just don't use at all, and maybe that suits management because they'd rather you didn't take time off from work anyway. (ex. application you use to claim time off from work)
Consider an intranet. If you have one password to log into all of your enterprise applications you are going to know how to log in and you will use all those applications.
If you have 20 different applications you use at work and you have 20 different logins, probably some of those services will end up in a state where you have to bug IT people to get a password reset when you ever need to use them. There will be others you just don't use at all, and maybe that suits management because they'd rather you didn't take time off from work anyway. (ex. application you use to claim time off from work)