I like the look of unfiished maple wood or when its oiled (like in certain rubbed furniture). I also noticed on some floor samples that I tried to scratch on purpose that the finished floor samples showed scratches worse than unfinished - or at least the scratches bothered me less.

Can you put in an unfinished maple floor and leave it that way? Will it accumulate more much dirt, stain worse, or get more damaged by humidity or moisture (the sides and bottom under the surface aren't finished anyway). Can I just apply some sort of wood oil periodically (not every day/week/month).

If the answer is yes, what other types of wood floors are amenable to installing and leaving unfinished or oiled?

As far as I know you can use a penetrating oil on any wood, and it is great as long as you maintain it. If you search this message board I'm sure you'll find discussions on products and techniques.
As for unfinished, I'm afraid you probably wouldn't like your floor very long if you left it so. Wood likes to absorb, so eventually it would find something damp & dirty to do just that. I suppose some of the very oily exotics might do ok for a while, but in the end I expect you'd still want to apply more oil.