[MunchkinMan]

1. The main difference is, as you say, the amount of Move it takes to go through them. Certain Classes and Races get to ignore those values. And Monsters can't generally go through Hidden and Locked doors.
2. A card in play must be face up.
3. The only cards that can be in play are Items, Races, Classes and cards that say they can be in play, so even if a DxM says it's usable once only, it can't be kept in play unless it also say so.