Google "Karel Robot", and have a look at the results: web pages about how a Java or C++ version of "Karel the Robot" can be used to teach (parts of) a first year university level in computer science. You'll find explanations on how important programming concepts can be taught using modern paradigms, object-oriented and polymorphism. All serious stuff.
Now, Rur-ple is always described as a fun way to teach programming to children. Nothing serious about that.
Granted, the lessons included so far (save for a few incomplete examples) do not touch "important modern programming concepts, like objects, and polymorphism." And granted that I wrote the lessons so that even children could learn programming by using Rur-ple; I even made some comments about this fact. It is (only) possible to do so because Python is used (instead of C++ or Java).
I guess I will have to wait until I finish writing the first 50 lessons or so before I can have people notice that Rur-ple could perhaps be taken more seriously by older children.
That's my rant for 2006!