There's so many....
Great personalization features (the ability to customize content based on list attributes dynamically).
A/B or multi-variant testing engines on send time, subject line, body
Strong reporting for open rates, ctr, geographics
Triggers/ series based email campaign kick offs to do automated programs
Native social sharing tools
Data output to larger CRM / analytics services for 360-degree profiling
Easy to use interface for management
List metrics and included non-responder supressions