Maybe this will help...
http://ask.yahoo.com/20020528.html
In a nutshell, thread-count is probably what most people would say in response to your question, however, that is not really the answer. A good name and quality materials (hence the good name) is what will distinguish one set of linens from another, much more so than thread count alone. Some of the cheapest junk I've ever seen had (at least supposed) high thread count.