An article was recently published that looks at evaluating First Programming Languages (FPL) the language to use for an introductory course of programming.
An existing issue is that formally assessing a programming language isn't really defined, with a lot of evidence being anecdotal. The proposed evaluation framework looks at technical and environmental feature sets. "The technical feature set covers the language theoretical aspects, whereas, the environmental feature set helps evaluating the external factors." These feature sets are covered in table 2 of the article (link to PDF) and consist of the following:
The article explains each of these points in details, and gives each of the languages being evaluated a rating based on this explanation, followed by a detailed explanation of how the scores of each rating can be compared this includes allowing an evaluator to weigh certain criteria they deem important against the others. As this is for choosing a language to teach someone to program with, different places will have different reasons and goals, so would want to weight things differently.
As the default weight settings do not conform to the original popularity index of the languages, so there should be a different weighting criterion. However, it is very hard to come up with a generic and correct weighting criterion. Therefore, the scoring function should be customizable and the user should be able to tune the weight of each feature based on her preferences. As an example, consider the fact that Ada holds 3rd position in overall scoring, but is not being considered among highly used FPLs as of now.
I'm currently teaching an intro to Computer Science course and we're using Python. It might not have been my first choice, but I sort of inherited the course.
My biggest complaints about python as a first language is that it doesn't enforce enough to really help the students understand what's going on under the covers. Some of the convenient things, I think make it easier, like the first week, but when you start writing slightly more complex programs, they become a hindrance.
Of course having done my undergrad programming work all in Java, I might not recommend Java either, but then I think about how Java actually makes you follow rules, so maybe that is a better choice.