As far as I know, Ruby works by evaluating AST (like Perl). No bytecode, i.e. pyc. This has speed implications.
Being addressed with YARV. I think this is one of the factors that don't actually make much of a difference.
Different GC approach (refcounting and mark-and-sweep) and its implication to C extensions need to be mentioned. Debunk "Python lacks real GC" myth. (Cyclic GC was there... since 2.0!)
Agreed, writing Ruby extensions is a lot easier than writing Python ones. This comes with a downside of course: Ruby's conservative GC isn't guaranteed to collect all objects. (There might be things on the stack which look like object references, but aren't.)
There's plans for having a generational GC in Ruby 2. This ought to help quite a bit with performance if done right.
Jython and IronPython need to be mentioned. I don't know much about JRuby.
JRuby seems stable. It's being used in jEdit for making a lot of magic work magically. Pretty cool stuff. There's a lot of Ruby.NET implementations and I'm working on one of them (getting sponsored by Google's Summer of Code) -- you can eventually expect a break through in that area, but it might still take some time.
Compare to Perl's taint mode.
ruby -T is exactly that. :)
Python includes parser module and compiler package to deal with Python sources. Ruby didn't, last time I checked. Also compare Perl's downright horrible B module -- but at least it's there.
Ruby comes with Ripper in the development branch AFAIK. That's a good way of parsing Ruby source code. There isn't an established and sexy parser framework just yet.
Get IPython which completely blows. One may stil argue irb is a part of core distribution and IPython is not.
Any features that IRB doesn't have?
Distutils. No, gem doesn't count, it is a package manager. (Contrast PHP's PEAR vs. PECL.) For building C extensions, I think distutils got many things right compared to Ruby's mkmf or Perl's MakeMaker, the most important being independence from Makefile. I am not up-to-date on Ruby/Perl for this. Corrections welcome.
Not too experienced here, but it can probably be solved with a custom Rakefile. See http://rake.rubyforge.org/