Features

Easy

Easy-to-use language similar to JavaScript+Rust with dynamic typing.
Tight integration with native Rust [functions] and [types][custom types], including [getters/setters], [methods][custom type] and [indexers].
Freely pass Rust variables/constants into a script via an external [Scope].
Easily call a script-defined function from Rust.
Very few additional dependencies (right now only num-traits to do checked arithmetic operations); for [no-std] builds, a number of additional dependencies are pulled in to provide for functionalities that used to be in std.

Fairly low compile-time overhead.
Fairly efficient evaluation (1 million iterations in 0.3 sec on a single core, 2.3 GHz Linux VM).
Scripts are [optimized][script optimization] (useful for template-based machine-generated scripts) for repeated evaluations.

Relatively little unsafe code (yes there are some for performance reasons, and most unsafe code is limited to one single source file, all with names starting with "unsafe_").

Sand-boxed - the scripting [Engine], if declared immutable, cannot mutate the containing environment unless explicitly permitted (e.g. via a RefCell).
Protected against malicious attacks (such as [stack-overflow][maximum call stack depth], [over-sized data][maximum length of strings], and [runaway scripts][maximum number of operations] etc.) that may come from untrusted third-party user-land scripts.
Track script evaluation [progress] and manually terminate a script run.

Re-entrant scripting [Engine] can be made Send + Sync (via the [sync] feature).
Support for [minimal builds] by excluding unneeded language [features].
Supports most build targets including no-std and [WASM].
Surgically [disable keywords and operators] to restrict the language.