The few important features I’ve used in membership plugins, in addition to forced login, are:
- Being able to assign different levels of access to users and being able to create these levels (gold, silver, etc.)
- Being able to specify how content is protected: by category, by tag, specific posts/pages, etc.
- Being able to protect parts of content using shortcode, although not as important.
- Integration with Classic Commerce would be important to sell memberships.
@anon98749105 was looking for a membership plugin. He might have more insight here.