What does the exclamation mark mean when approaching a door?

Solution 1:

Sometimes they become openable doors through the story (e.g. entrance/exit in Dead City), sometimes you get a keycard or key to open them (e.g. Gearhead Vault in Subway), sometimes they can be opened by shooting gas tanks behind them (e.g. Wasted Garage) or similar.

However, mostly they're just part of the wall that looks like a door - there seems to be more 'doors' like that than any other.

Solution 2:

It means that the door is not openable at all. A red hand icon would mean that the door is openable via other means (key, switch, explosive). I suspect this was put in so that future DLC may change this, but that is only my suspicion.

Solution 3:

They place it so that you can see that you are indeed properly positioned at a door and it is indeed a door but that it cannot be opened. It is a different style then games where you see doors and you try to get up against it and open it but it just is part of the environment. It's more of an aesthetic and design decision then anything practical, other than the fact that you know you are located properly in place of a door. Makes you think that there is a room behind it, and more things, gives the place a more fleshed-out feel, in my opinion.

Also like mentioned by others, since it isn't a piece of wall that appears like a door, and is apparently programmed as a door "entity", from a programming standpoint it would make things easier to implement the ability to open any of these at a later date through a DLC or occurrence within the game.