A Resource-STS (others call it RP-STS or federation gateway) is a necessity for non-trivial federated identity scenarios. ADFS v2 does an excellent job in fulfilling that role – but (as of now) you have to run ADFS on-premise.
The Azure Access Control Service is a Resource-STS in the cloud (with all the usual scalability/availability) promises. Unfortunately a lot of (the more interesting) features in ACS v1 had to be cut due to constrained time/resources.
The good news is that ACS v2 is now in CTP and brings back a lot of the missing features (like WS* support) and adds some really sweet new ones (out of the box federation with Google, Facebook, LiveID – and OpenId in general). You can read about the details here.
On a related note – ACS v2 works out of the box with StarterSTS – simply choose the ADFS v2 option and point the management portal to the StarterSTS WS-Federation metadata endpoint. Have fun ;)