When you say "I got off the bus," no particular emphasis is added, IMO. It's just something you did as part of a broader context. For example: I got off the bus on Main and Third and walked towards the restaurant.
But when you say "I stepped out of the bus," you're adding a certain degree of emphasis and it becomes a more or less important part of whatever action you're describing: I (slowly/nervously/etc.) stepped out of the bus and looked around to see if she was there.
At least that's how I would see it.
Edit: I didn't see #6 when I started.
But one steps down out of a bus, if anything. - That may be true in a literal sense, but I don't think a lot of people would actually say "down" in that context; "out of" by itself is usually enough, IMO.
FWIW, google tells me that there are 592,000 hits for "I stepped out of the bus" vs. 6 hits for "I stepped down out of the bus."
