To me nostalgia isn't much of a reason to replay games, but remember that's just me. I see more value in replaying games for the enjoyment overall, in addition to reliving some memories, good or bad.
Erm... Isn't that kind of what nostalgia is...?
Anyway, I can't speak for everyone, but nostalgia matters a great deal, in my opinion. Older games that I've played hold a place in my heart, and I can always return to them and play them many times over. Those games possess certain qualities that I don't find in newer games. The absence [or lack] of said qualities in newer games often leads to nostalgia and causes me to recall a time when said qualities were more prevalent.
While on the other hand, newer games might also possess certain qualities that only serve to
remind me of older games. To be honest - above all else - nostalgia is the reason as to why
Ocarina of Time is still my favorite game in the Zelda series. It possesses qualities that can never be replaced by anything a newer game in the series has to offer. I always find myself comparing newer games [in the series] to OoT in an attempt at finding out what the former did better than the latter; this seems to happen automatically in spite of any improvements, additions, or innovations a new game has. More often that not, there is a sentimental value attached to the older games as well.
From my experience, sentimental value has dominated above all else. Sentimental value and nostalgia go hand and hand.