Don't understand how Linux < AIX ? (Score 2, Interesting)

Everyone in this discussion is talking about how Linux is not quite/ not yet up to the mark of a commercial unix variant. I have occasionally used Unixes ( AIX/Solaris/Linux) mostly as a programmer using POSIX/Unix APIs and haven't found much difference(other than the fact that the documents for linux are much better and charming).I don't really know much about "enterprise quality , mission critical" operating system features. So assuming these people are right, where do you get such comparisions / technical information /feature lists ? Any links , mailing list etc will be greatly appreciated. And while we are at it , can anyone please explain why the hell I can't print a < sign in my subject header ?