My suggestions based on my views and what has come so far:
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Data privacy and ownership - it’s 100% your data in the sense that the complete data is on your local machine. No on-line account required, no ads, no tracking. Notes are created using Markdown. No proprietory formats.
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Has an optional “web-clipper” for Firefox and Chrome. Just a couple of clicks in your browser can turn a web page into a note.
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Option to sync with cloud services as well as your own Nextcloud server or even your file system. Lets you access and edit your notes on all your devices (Windows, Mac, Linux, IOS, Android). Also you can sync using End-to-End Encryption (E2EE) so that notes are encrypted on the storage server. Only you can read them.
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It’s easy to use (or it tries to be) and can be extended with Markdown plugins. Already supports advanced Markdown tables, tables of content, maths formulae, Mermaid diagrams and more.
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Notes are presented in an easy to read format but more advanced or adventurous users can customise Joplin using CSS. Change how the notes look when viewed on the screen or printed, or both! You can even customise how the editor looks. It’s totally up to you.
Whilst I see why the “…open source with an active community…” point is made, an open source project can fold just a quickly as a commercial one. It’s not guaranteed that someone will fork the project and take it forward. Also this is more a benefit of FOSS in general and not Joplin in particular.