Format: Discord's main strength is chat-style messages, not forum-style discussion threads, like Reddit and Lemmy. Discord groups with more than a few dozen active users can quickly become disorganized.
Barrier to entry: Content on Discord is inaccessible unless you have a Discord account, while almost all content on Reddit and Lemmy is available without registration.
Discoverability: Google (and other search engines) index Reddit and Lemmy, and relevant threads show up in searches. Discord content cannot be indexed, and won't show up in searches.
Censorship: A Discord community is ultimately still controlled by a single Big Tech company, which can delete your community on a whim if they so choose. Lemmy, being a distributed social network, is inherently resistant to censorship.

Any further suggestions or tweaks?