I would definitely second what Sean says. What we have in Mahout is a collection of algorithms that we know will scale. Mahout is not about being the broadest collection of learning algorithms. It is about having scalable algorithms that are reliable in scaling situations.
If you want a broad selection of algorithms with much less attention paid to scalability and deployability, then use R.
If you think we really, really need some algorithm in Mahout and you know how it should be implemented to be scalable, please do come over to
[email protected] and let's talk about it!