See https://premium.wpmudev.org/blog/brilliant-wordpress-email-newsletter-plugin/#comments for my suggestions but in general:
A/B split testing.
Unlimited auto-responders & tracks.
History tracking of what’s been sent to whom and how they’ve responded.
Triggered events based on custom field content. (Automatically send a personalized birthday note to someone on their birthday.)
Ability to maintain separate lists and can then segment those lists into sub-parts.
Ability to set up triggers for each different list and segment based on specific criteria.
Ability to “move” people from one list or segment to another automatically based on their actions.
Ability to easily import Email addresses & be sure they are de-duped.
I put "commercial" in quotes because Interspire has virtually stopped development of their solution because they moved their focus onto their shopping cart business.
Meanwhile, Octeth is less visually polished and their knowledgebase is so bad, it doesn't have working pics and the instructions are old... (They claim to be fixing this.)