I really like ACS2. One feature that is very useful is home realm discovery. ACS provides a Nascar style list as well as discovery based on email addresses. You can take control of the home realm selection process yourself by downloading the JSON feed or by manually setting the home realm parameter.
Plenty of options – the only option missing is turning it off…
In other words, when you setup your ACS namespace and realm and register identity provider, there is no way to keep the list of identity providers secret. An interested “user” can always retrieve all registered identity provider (using the browser or download the JSON feed).
This may not be an issue with web identity providers, but when you use ACS to federate with customers or business partners, you maybe don’t want to disclose that list to the public (or to other customers). This is an adoption blocker for certain situations.
I hope this feature will be added soon.
In addition I would also like to see a feature I call “home realm aliases”. Some random string that I can use as a whr parameter instead of using the real issuer URI.