One of my favorite feature of IIS7 (well, there a lot) is that user profiles are now loaded for the worker process accounts. Why is this important?
A lot of the Windows functionality (and in my case – security features) depend on loaded profiles, e.g. DPAPI user keys and per user certificate/key stores. Prior to IIS7 you were always forced to use DPAPI machine keys (with application defined entropy) or to store application certificates in the machine store (and manually ACLing the private keys). This also affects the protected configuration feature which relies either on DPAPI keys or the key store. Per application pool profiles (and thus keys) makes isolating applications on a server now much easier.
Another interesting fact is that IIS7 injects the required group (called IIS_IUSRS) into the worker process account – in IIS6 you had to manually add the worker process account into IIS_WPG group.
To demonstrate all this I hacked a little proof of concept web site together that shows the worker process groups and uses the ProtectedData APIs, protected configuration and the X509Store class to query the user certificate store. Nice!