It seems a lot of the "magic" with Ruby comes from Rails, and I would say that teaching a new language using a framework is probably going to be pretty overwhelming to any beginner.
One thing I would worry about with Ruby as a teaching language is that the syntax is somewhat unique. I don't think this is a terrible thing, the basic structure is similar once you get past the syntax, but I think it can trip people up if it's their first language, and then they move on to Java/C++/Perl/Python/PHP which seem to follow more of a continuity.
I think, from my experience, I actually think learning a couple different languages at once can be really advantageous. I pretty much learned Pascal, C and Assembly at the same time, working in the demoscene, and linking them all together.
The important part of programming isn't the syntax, it's the structure and the concepts. You can always look in the manual for syntax, but structure and concepts are the hardest parts to grok.
So maybe my suggestion is to have side-notes for the examples that say, "here's how you do this in PHP, here's how you do it in Java, here's a breakdown of the differences and similarities."
On the other hand, I may be completely wrong, because I'm biased based on how I learned programming, and not everyone learns that way.
As a TA for an intro to programming class taught using Java, I wish, wish, wish we were using Ruby instead. I spend 70% of my time teaching people Java's obtuse syntax, rather than teaching the underlying concepts. Once a programmer has developed the concepts, she can code in nearly any language, but she really has to learn the concepts. Ruby's syntax is straightforward enough that little kids get it down in days, so students can actually learn computer science rather than Java science.
It depends what you're learning. If all you're learning is to memorise blocks of code, then Ruby is probably easier to learn because there's less of it. If you're trying to learn what a programming language is actually doing, I don't think Java is any more difficult than Ruby. If anything, most Java code is a lot more clear about exactly what is going on.
FWIW, I learnt and tutored Java at University and have recently been learning Ruby and Ruby on Rails for fun.
We also can't underestimate how much of an effect being familiar with a certain style and syntax has an effect on our judgment of languages which break from that style.
Since I started with Pascal and C++, and then branched to Perl and PHP, the first time I looked at Ruby code, it looked like gibberish. But Java and Google Go immediately looked very familiar to me.
I've since learned Ruby better, and it's kind of come into focus, but I think that's definitely something to keep in mind when we're teaching programming to others.
I think the hallmark of any good teaching language is the ability to get up-and-running quickly, and Ruby (and PHP and Perl and Python and scripted languages in general) have that over Java, C++, and others.
But I think it's important to compare and contrast languages for just that reason, so that when moving to a language which requires a lot of preparation before you can even echo "Hello, World", the student doesn't get frustrated.
One thing I would worry about with Ruby as a teaching language is that the syntax is somewhat unique. I don't think this is a terrible thing, the basic structure is similar once you get past the syntax, but I think it can trip people up if it's their first language, and then they move on to Java/C++/Perl/Python/PHP which seem to follow more of a continuity.
I think, from my experience, I actually think learning a couple different languages at once can be really advantageous. I pretty much learned Pascal, C and Assembly at the same time, working in the demoscene, and linking them all together.
The important part of programming isn't the syntax, it's the structure and the concepts. You can always look in the manual for syntax, but structure and concepts are the hardest parts to grok.
So maybe my suggestion is to have side-notes for the examples that say, "here's how you do this in PHP, here's how you do it in Java, here's a breakdown of the differences and similarities."
On the other hand, I may be completely wrong, because I'm biased based on how I learned programming, and not everyone learns that way.
With all that said, this is a pretty great piece.