Yep! Sound design is an art. Obviously it depends on your film (Looney Tunes has very different sounds than your average horror film) But as has been said, the more detailed you get, the more others will take note.

If there's one brickfilmer that nails sound design every single time, it's ForlornCreature. Some of his behind the scenes videos have information on how he does it.

To answer your questions:

One of the biggest lessons I learned was the importance of ambient/atmospheric noise. While background music can help hide it, if you only include footsteps and other major noises, they will feel disconnected and strange. The real world constantly has sound bouncing around from a thousand different sources. Adding even a little to the background goes a long way in beefing up the entire soundscape.

For this film, the sound would be A. The obvious stuff like footsteps, dialog, and gunshots B. The somewhat background noises (Usually the droning lights) C. One or two background noises (Cars, wind, or that cool whooshing thing) D. The music. That seemed to work out well for that film.

As for realism, personally, I do footsteps, but only clothing if it's something obvious. (A character with a cape or backpack, for example.) But the more my film's sound has reached for live-action levels of detail, the more they've been praised for that. It sets films apart and can give them that extra edge of awesome.

I'd say go for as detailed and realistic as you can. (Within reason)