My take on it is that if they are going to block los as part of the model then they have to be considdered part of the model for placement and movement reasons. Besides, if your ork EVER tried to run across my access ramp, the machine spirit would close the flaps up and trap you inside untill my storm bolter had you dead!
GW don't make the rules for this sort of thing EXACT, because that would make every codex 10 pages thicker, and makes rules discussion a little less interesting.
Ultimately it's up to the people who are playing to decide.
I look at it like this:
Drop pod comes down, doors closed.
Drop pod opens doors, any doors that are obstructed stay closed until they can open free of obstruction.
Marines come out of the opened doors, Marines that would have exited a closed door just climb over their harness bit.
Pod stays where it is, all doors open if possible, and shoots.
If an obstruction moves out of the way (infantry, or vehicles next to it), then the doors will open if they can.
People can walk over the doors, as considering it a part of the hull is kinda stupid, and since it's something that can change over time, really stupid.
The only part of this that can really be argued about with this process is whether or not opening the doors is even required.
Obviously someone who glues the doors on can't do anything about it, in which case you just use your imagination.
It is my opinion that all the doors are considered down for LOS purposes, any doors which remain up are ignored.
You can practically always shoot through the Pod itself conferring a save, if the unit behind it is tall enough.
This pretty much means that unless it's elevated, it has to be about the size of a Dreadnought to be visible; of course I don't have the models to reference, but I think that's about accurate.
The Drop Pod for Dreadnoughts is a different matter entirely.
Forge World has a great one available, but obviously that's not really an option for everyone.
For people who just want to use the normal Pod kit, I suggest either doing a conversion, or merely removing the centre part of the inside (how else could you fit a Dreadnought in there?)
This will make it possible to see completely through, although as you're firing THROUGH a model, it'll still confer a cover save.