- Centralized control over identity: Bluesky holds user keys custodially, meaning your account is only portable as long as Bluesky allows it. By contrast, ActivityPub lets you fully control your identity and move servers seamlessly.

- No private communication: Bluesky's direct messages are centralized, unencrypted, and routed through its servers. ActivityPub-based platforms support private interactions while keeping your data decentralized.

- Public blocks: On Bluesky, blocks are public records, exposing user interactions and potentially enabling harassment. Mastodon and other ActivityPub platforms respect user privacy with more thoughtful moderation tools.

- Limited federation: While Bluesky is technically federated, its architecture forces relays to store everything, favoring centralization. ActivityPub excels here, enabling a vibrant ecosystem of thousands of independent, lightweight instances.

- Dependency on domains: User handles on Bluesky rely on DNS (e.g., user.bsky.social), which is inherently centralized. ActivityPub’s flexible architecture doesn’t tie identities to domains in the same way, offering more resilience.

- Everything is public: Bluesky's architecture assumes all content is public, limiting privacy-first interactions. Mastodon and similar platforms provide flexible visibility options for posts, from public to followers-only and direct messages.

Reference: https://dustycloud.org/blog/how-decentralized-is-bluesky/

Sure, ActivityPub isn’t perfect. But ActivityPub (powering Mastodon, PeerTube, and more) has proven its value by enabling a truly decentralized ecosystem, with lightweight, scalable tools and user-centric design.

Improving upon ActivityPub holds the promise of a better decentralized future—one that doesn’t just mimic the old centralized systems we aim to move away from but builds something fundamentally better.