How does suppression work? That used to be a major paint point with mailman.
How do you stop spambots from harvesting the email addresses? (I guess “not at all” is a valid answer there as mailman pretty much does that.)
Other than that, a read-only archive seems like a no-brainer. We could even have a separate Discourse instance for archiving all mailing lists. There are external tools for mailing list search and threading (gt.net, mail-archive.com, marc.info etc), but they are not great, tend to disappear over time, and we have no control over them. An archive where you could claim your email address and a profile would be pretty cool in itself (even if currently you need a separate account for each email address, annoyingly). And then stuff like Onebox would probably do more good than harm, on net. (Now if only Phabricator or MediaWiki actually supported Onebox!)
Some people were hoping Mailman 3 would fix the archives though (T130554), it has a lot of the obvious features like search or threading, and even some social stuff like message up/downvoting. Not sure where that effort is at.