I've started some work on SAML 2.0 usability from a service provider's point of view. The more complex architectures you make with SAML 2.0, often the usability suffers. In particular series of "Where are you from" interfaces, and service provider not already know about an existing SSO sessions are the two main problems I try to solve.
SAML 2.0 introduced a number of features not available in previous versions of the specification, including:
Pseudonyms – SAML 2.0 defines how an opaque pseudo-random identifier with no discenible correspondence with meaningful identifiers (for example, emails or account names) can be used between providers to represent principals. Pseudonyms are a key privacy-enabling technology because they inhibit collusion between multiple providers (as would be possible with a global identifier such as an email address).