It has to be a subject that interests me in general and/or that I'm looking for at any given time; and it has to be well written. If I'm looking for "how to make brownies" I only want info on how to make brownies. If I'm looking for "reading-in-general" (and regardless of the kind of subject or style of piece) I'm looking for some kind of substance behind the material (whether the material is information, humor, fluff, or "whatever").
For the most part, I don't see even the most well written, informative, articles as "great". The best I think articles can be is "good" or "solid". For me, an excellent opinion piece or "ideas" piece could potentially be "great", but even the best of those are something I'd most often only call "good" or "excellent" at best.
I don't happen to read most stories or any fiction; but when all is said and done, I really only see "great" as a word that could potentially be applied to story telling - whether fiction or non-fiction. Even so, however, I think "good", "solid", and/or "excellent" are enough to make most non-fiction/non-creative pieces of writing "worth reading". Also, my thinking is that if a subject can be presented/is being presented on authority sites somewhere else, what I'm looking for on a site like this one is something more, or something different from, what is being presented well on those authority/expert sites (because if I can read the same thing there as I can read here, I'm going with "there"). I'd rather read something that questions/further examines "conventional wisdom" (even expert wisdom) than something that echos the same old stuff that's presented somewhere else (and with more expertise in the subject).
Then again, I'm not as picky as I may make myself seem. Basically, I like to read most ideas/opinions presented by anyone who expresses them well and has a solid understanding of his subject. That's the thing: A lot of people believe that "Internet readers" ONLY want information. There's no doubt people who search most often want information; but I, personally, am pretty much "all informationed out" at this stage in my life and with having the Internet available for as long as we now have. I like to read original ideas and thinking (but they have to be well informed ideas and thinking - not just stuff people willy-nilly offer "out-of-the-clear-blue-sky" opinions on regardless of their own lack of familiarity with/solid exposure to the subject.