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I haven't studied them really closely yet, but I noticed under their listing (they're number one out of 180 million results) Google displays links to four of their inside pages and has a 'more results from this site' link to boot.
Have they managed to become a whatchamacallit (an authority site?) and that's why they get the extra bells and whistles in their SERP listing?
Thanks in advance,
One thing to notice is that just having a number 1 position doesn't do it -- the search term itself is also critical. For example, a search on 'webmasterworld' shows the extended description, but a search on 'webmaster' does not. So we are considered THE authority on 'webmasterworld' (of course) but not on 'webmaster'.
Still there's more going on here than a mere domain name match. Bottom line is that those extra links are generated by algorithm and not manually. And they are given to strong sites, with clear authority and high quality signals for the given search term.