* Some things should be SUPER easy, hard things should be possible, nothing has to be fast or maintainable
* Fast feedback cycles between writing a thing and seeing if it works, with easily-visible results. We want to introduce students to the phenomenal magical power of coding, because that will provide intrinsic motivation to keep going when things are hard.
* Supports encapsulation and recursion: these are the hardest concepts for most students to grasp, so introducing them early and insisting students use them is valuable.
* Good code should be pretty, ugly code should be ugly: I don't care if it's possible to write terrible code, I care if readable code is obviously readable. Students need to be able to start developing an aesthetic sense of code quality right away, but shouldn't have to write clean code in order to get something working.
* No meaningful whitespace or magical numbers of characters: these are often confusing to people not used to working with computers because in other places we use language those things don't matter.
* Good IDE support: most students aren't going to be used to working on the command line, and introducing version control is a more gentle introduction. This helps with explorability and getting students over the hurdle of understanding that they aren't writing prose: they are building a world with it's own internal rules system.
* Easy unit testing: it's hard enough to teach unit testing even at its easiest, but it is incredibly valuable to start with it because it introduces the idea of interfaces and encapsulation in a particularly tangible way. It can also help students learn to evolve software in small, safe steps.
* Publicly-visible well-written code bases: reading code is just as important as writing code when learning.
* An active, supportive, anti-sexist community: I want students to be able to feel like they belong when they go looking on line for information about what they are doing.
* Doesn't try to be clever or optimize for experienced user productivity: ideally, I should be able to tell a story about the language in a single sentence. "Everything is an object" or "everything inside the parentheses gets run together" or "we send objects messages" are ways to bootrstrap a mental model of the language (consider, for example, how Bootstrap explains LISP to teach students algebra: http://www.bootstrapworld.org/materials/spring2017/courses/algebra/units/unit1/index.html#lesson_Brainstorming5496)
Note that many of these are different things than I look for in a production language. I want students to make mistakes that help them learn, so protecting them from those mistakes isn't useful or helpful. They aren't going to be working on large code bases, so libraries, package management and scalability aren't important. No language is perfect on all of these dimensions, but some are definitely better than others.