On the subject of Cargo, I'd like to be able to show and run Cargo examples. Play can be fixed to support it perhaps. I don't want to explain Cargo.toml on the Rust home page. Having to explain that the example doesn't work without pairing it with another file, and one that is not interesting or relevant to demonstrate the beauty of the Rust language, is extremely undesirable to me. Cargo should have a subcommand or plugin that will figure out what crates you want from the source so we can say 'Run this as main.rs locally with cargo blah'.
cc @Lurkin about play not dealing with Cargo
Edit: cargo-script does something close to what I'd expect for demoing simple cargo-ified source. The only annotation in requires is // cargo-deps.