Metal Gear Solid has such a unique mood compared to the others. Everything's so cold and cavernous, with a real eerie, echoey vibe to it.
Speaking of mood:
In general I think most tracker music has something to it that modern music is missing, propably the fact that it's created with love and passion, and not money in mind.I don't know about that last bit, but I definitely agree that tracker music has a really unique sound. It's a big part of why Unreal, Tournament and Deus Ex sound so great. Ion Maiden will be using it too, which is just awesome.
I think the technical term is Friggin Huge. ;DI checked it out, boy you weren't kidding! Amazing.