I have a unique perspective on this I think, since I am just now playing Shenmue for the first time! After years of wanting to play the game (ever since I saw it on reviewed on Tech TV when it came out), I finally bought a Dreamcast in order to play the game. I'm really blown away with this game. So much about this game feels ahead of it's time, to the extent that I'm left feeling that some of the things being done by modern games like HEAVY RAIN where being done by Shenmue ten years earlier quite well.
The scope of the game and the depth of it are so impressive to me. I think it's aged magnficently because rather than focusing on MACRO-DETAIL (like say... GTA III, or Elder Scrolls, which attempted to simulate a giant world, but with a lack of detail in individual elements of that world) it focuses on MICRO-DETAIL. Every little thing you pick up is thoughtfully constructed. It's such an unusual game. It joins the rare club of older games (like Silent Hill 2, for example) that I feel haven't aged at all. There are still no other games quite like Shenmue.
I'm in love with this game. I have Shenmue II ready to go on the original Xbox when I beat this one.