I have been working with ISA Server for about couple of years and by looking at the new release of ISA Server 2006, below i have mentioned down some new key features
Share Point Portal Server Publishing wizard - ISA 2006 is designed to provide secure remote access to Share Point Portal Servers.
Full support for Exchange Server 2007 - With ISA 2006 its very simple to makes publishing of Exchange easier than ever.
Forms-based Authentication - ISA 2006 allows you to use forms-based authentication for any type of Web publishing scenario
Single Sign-on - If multiple Web sites belong to the same domain, and are published by the same Web listener, then users will not be required to reauthenticate and cached credentials are used.
Branch office VPN connectivity Wizard - ISA 2006 now has a branch office deployment wizard, that enables you to connect your branch head office to your remote office with a secure link.
Enhanced Delegation of Authentication support - ISA 2006 enhances support for authentication delegation by enabling credentials to be delegated as Kerberos, Integrated, Negotiate or basic.
Flood Resiliency - ISA 2006 includes built in mechanism to prevent exhaustion of non-paged pool memory so that even when under heavy denial of service type worm or DNS flood attacks, the ISA 2006 firewall will be able to stand up.
Enhanced remediation during attack - ISA 2006 has updated stateful packet inspection and IDS/IPS functionality.
Support for LDAP authentication - ISA Server 2006 can authenticate to an Active Directory without needing to be a member of the domain.
BITS caching - ISA Server 2006 provides the caching mechanism for data received through BITS.
Web Publishing load balancing - ISA 2006 automatically balances request streams among ISA array members.
HTTP compression - ISA 2006 performs HTTP compression that reduces file size by using algorithms to eliminate redundant data during transmission of HTTP packets.
Quality of Service - A new packet prioritization functionality is incorporated into ISA 2006, which scans the URL or domain and assigns a packet priority using Diffserv bits.
Integrated support for Password changes on logon form - ISA 2006 adds the ability for a user to change his password right in the log on form with no special configuration.
Improved Alerting - ISA 2006 adds a number of new alerts that help information the ISA administrator of configuration issues, certificate issue, security issues, and threat triggers.