I'm not as engaged on Server Fault as I was when the site went public, but I don't have any feeling that the "quality" of the questions (both as a measurement of their intellectual merit, or as an assessment about their subject area) has changed dramatically. I feel like there's been a decrease in the number of questions and traffic to the site (or, perhaps, just a lack of an increase), but I'm assuming that the corpus of already-answered questions is helping some people get answers w/o having to ask new questions.
I'd direct you to the question Brandon mentions in his comment (How do we grow the Server Fault and Super User communities?How do we grow the Server Fault and Super User communities?) regarding some ideas on getting "new eyes" on the site.
I do think that the "problem domain" that SF is oriented towards is smaller, by far, than the SO problem domain. By nature, most sysadmins are using COTS tools and operating systems, and typically these tools are configured in "best practice" type configurations. There are a lot of possible permutations, but many don't produce productive results. If my work as a contract sysadmin has taught me anything, in excess of 90% of the technical details of computer networks are the same from network-to-network. Those that are radically different are typically pathological cases where somebody didn't know what they were doing setting it up.