Just some basic particulars (if you can call them that): he is not in the habit of carefully exploring alternative views to his own (instead he ridicules them without attempting to understand them); he seems to write as if both his deductions and assumptions are obvious rather than question them; he almost never tackles more current actual philosophical ideas (rather than occasionally paint them in an "obviously" ridiculous light by completely misunderstanding them); most importantly, he seems to present a definitive answer after definitive answer rather than seriously and systematically explore questions.

That is not to say that I don't occasionally agree with his conclusions, but his process lacks the originality and constant self-doubt that makes philosophy so entertaining and interesting. Instead, it offers dismissal or certainty -- stuff that may make religion or blog posts entertaining. But that's not philosophy.

Besides, my vague insults are pretty much the common view (which is actually far more dismissive than I have been). It is anyone in the small minority who considers Yudkowsky a philosopher that requires substantive justification. In any case, even if you find his writings convincing, Yudkowsky would still make for a very bad foray into philosophy, because he tends to make up his own nomenclature rather than use accepted one (and present age-old ideas as his own). His fondness of definitive answers and ridicule of others means his work mostly serves to confuse rather than enlighten anyone who is interested in what philosophers do, or what current interesting debates are taking place. Even if you disagree with all that, you would surely agree that a fringe thinker, unrecognized by the academic community and who's not recognized to have made a single original contribution to the field (pretty much by anyone) is probably not the best guide for a beginner.