License Requirements for AnyConnect Connections:
http://www.cisco.com/en/US/docs/security/asa/asa82/configuration/guide/svc.html#wp1079517
Basically, with AnyConnect Essentials license you lose the functionality of Clientless WEBVPN (the portal page) and of Cisco Secure Desktop (CSD). You need to also read between the lines when we say AnyConnect doesnt support CSD -- Hostscan is part of CSD. Hostscan is the part of CSD that gathers information about the connecting computers to be used in Dynamic Access Policies (DAP). So having an AnyConnect Essentials license would really limit what you can do with DAP (which is one of my favorite features).
The Premium license doesn't have any of these restrictions.