The style, the overall feel. I suppose that's primarially the music. Story has some part in it, but rather than because it is a good or bad story, because of the style of the story. Thing is, the story is rather cliche in a lot of regards, and not anything spectacular. So it's the overall style, into which the story as well as characters play (and music, too), that makes it what it is. On each seperate thing, a lot of things beat it - music, Cross probably does; story, something like Knights of the Old Republic; writing/dialogue, say FFXII. But what made Trigger what it was, and so endearing, was the stylistic interplay between these. It's the same way we can watch a classic movie from the 50s which has inferior effects, inferior writing, inferior music, and maybe even inferior acting, and still enjoy it as much or more than a modern movie. This is especially so if it happens to be a B-type movie, and that's the sort of style that Trigger follows after. It was good in the same way that a movie (and here I'm citing a modern one) like Independance Day was good.
So I'll respectfully disagree especially with those that say 'story' because, even if we assume Aristotle is right and that plot is the most important aspect of a story, still Trigger wasn't anything too special in that regard. Those feelings of nostalgia one gets when one recalls it aren't brought about by a good story. They're memories of style. Partially story, partially characters, partially music, etc., but mostly as they work towards the overall power that is the style.
