See this useful post for features and ideas

Thanks for sharing the tip, a very useful overview. As a simplenews developer I recognize a lot of frequently requested functionalities in this summary.

Add to this the Simplenews Template module:
Simplenews Template is a Drupal module that extends the Simplenews module by providing a themable template with configurable header, footer and style. Header, footer and style are configurable for each newsletter independently. Simplenews Template can with advantage be used in conjunction with RelatedContent.

Feature requests in this post: "Not much to fault this module, but if anything it would be good to see some features to improve scalability such as batch sending and send throttling. It would also be good if you could set an newsletter to include new/updated content from selectable content types if you wanted automated newsletter generation, but that is not a biggy in my view."

'batch sending and send throttling' was and is included.
'include new/updated content from selectable content types' is what you can do with Views, views_insert and (re)send the module using triggers and actions (currently D6 only).