Yeah, some games do have devoted fans but I genuinely think the core Shenmue fans are different.
I think in some cases it has had a life changing effect on people, people go to Japan and get interested in the culture etc
The story is good but I think it is the atmosphere of the games and the look and feel that really made it stick with people. The tranquil ambient music to the well thought out subtleties that you don't consciously notice. It's hard to describe. There's just something different.
Someone summed it up saying it's like a religion to some fans and I think this is a big part of the reason Shenmue III was announced.
It's just a game but to many people for whatever reason it is more than that..