Small. The number of features is highly limited in order to ease a userbase bootstrap.
Experimental. Use at your own risk.
Specific purpose. That could change, but not too early.
Purely functional. Just because.
Simple. Only includes easymost features, for a short learning curve.
Readable. Because it helps the learning curve.
Customizable. It is designed with the goal of letting advanced users read the syntax they prefer (i.e. bracket based, white space based, parens based) while everyone is required to write a unique syntax (text based).
Extensible. It features a mechanism for custom extensions.
Implementation agnostic. There is no, and there should not be, such thing as a reference implementation. The commons should only exist as a spec.
Free. The spec should be public domain, CC0, GNU FDL, or anyway similar.
Open source. Some free to use open source implementation should exist, using licenses approved by both the Open Source Initiative and the Free Software Foundation.
There are plans to fill out and publish an instance of the "Programming Language Checklist by Colin McMillen, Jason Reed, and Elly Fong-Jones".