I dunno. What's the diaper equivalent of fahrvergnügen? LOL.
But I'm kind of serious, actually. I'm always tempted to answer questions like this with things like "babyish" and "feminine," and indeed those are contributing factors, but there also plenty of diapers that
are those things and yet hold zero appeal (for me). As well, there are things like prefolds that are strongly babyish to me because of my particular childhood experiences, but aren't innately babyish in ways younger people can easily relate to. Basically, it all comes down to: I'll know it when I see/wear/use it.
Despite that, my preferences are kind of all over the place, spanning disposables and all varieties of cloth diapers, so in practice I'm not exactly hard to please. It would probably be simpler to list things that are complete turn-offs for me, and there's really only one: Anything overtly medical in appearance.