Overview
The service pack includes the following new functionality and feature improvements:

New Reports
• The new Site Activity report displays a report showing the data transfer between users and specific websites for any user.

Error Pages
• A new look and feel has been created for error pages.
• Error pages can be more easily customized and can include embedded objects.

Kerberos Authentication
• You can now use Kerberos authentication when you deploy an array using network load balancing (NLB).

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For more Info, check the articles below:

Five reasons you should apply Forefront TMG 2010 SP2

E-Mail Protection fixes on Forefront TMG 2010 SP2