The gist of this would be:
1. It has a larger, more committed team than J. Random Developer. The core rust team promotes this as good place for Rust enthusiasts to contribute if they're not compiler specialists.
2. Everything added to the crate has similar naming conventions, argument order, intermediary structures, package hierarchy, etc. Basically an experienced user should have some chance of correctly guessing how to call a function they've never used before but hope is in the library. (The Java library does this - I just correctly guessed one of the ways to call java.awt.Graphics.drawPolygon without being sure it exists.)
3. There should only one of everything in the crate. (Java does not do this - awt vs swing for example.)
4. The extended standard library absorbs crates that are de-facto standards and no longer receiving active development / no longer need active development. It may rename some stuff on the way to fit point 2.
And some examples of crates in a current project that I thought could stand to be in such a library, either now or eventually, are dotenv, rust-crypto, serde, and env_logger.