I think the ease of use and organization are a big part of it. There are a LOT of chess sites, some have a lot better players and a lot more 'action' than here - but the interface sucks. Here you not only get good chess, but its cake easy to use.
The interaction and content are really good too. There is a steady stream of articles and news, good authors, plus all of the bonus training material.
Chess.com does not have as many GMs playing as ICC, not as many deep articles as chesscafe, not as much news as chessbase, several sites have good tactics engines but not the other training aids, nobody has anywhere close to the combination of play-content-train-news that chess.com has.
(the forums are nice too!)
My question is why is chess.com so addicting. It is an interesting question and inevitably many people will have their own reasons. I, for one use it to make friends in the 'chess' world; and of course to improve my game. Does anyone else have different views/theories as to why people use this website so much? The website has over 2 million users worldwide. Another possible aspect could be that this is a very singular site which has very few rivals anywhere and the places where you can find similar positive points are requesting fees. Anyone else out there have any views??? Regards Wealybin