However, I stay clear of Java or Visual Studio only people. They have a truly warped and unrepairable mindset.
Stay clear of anyone who is [anything]-only.
Anyone who will only use one language will warp all problems to that language -- and worse, warp all solutions to only those that they don't have to think about. It doesn't matter if the language is Java, C#, C++, C, Perl, Python, Ruby, or COBOL. If they are only willing to code in one language, let them go.
Nearly every accredited university offers "language survey" courses. This is where a CS degree can be useful -- the graduates have, in theory, been exposed to other languages. Bring this up in the interview. See if they can articulate the tradeoffs of various languages.
Entirely-self-taught developers often require a lot of basic remedial training. I'd suggest investing in them only if they will spend their evenings completing a CS degree. For an intelligent and skilled person, this isn't terribly difficult. The ones to be careful with are the "Meh, I can't be bothered to obtain/complete a degree." types. They might be intelligent, and they might be skilled. But their ego is going to make a lot of work for everyone else, as that can't-be-bothered attitude is a sign.
(Yes, there are lots of people who could only possibly succeed in an academic environment where the problem is carefully structured to be completed in five weeks by a mediocre and distracted person. This is where "what do you do in your free time?" comes in useful. One of the best teams I've ever worked on had "What are the last three books you've read for pleasure, and when?" as a key interview question.)
As for the 10% effect ... Sturgeon's Law.