wen you look trough a door diagonally you can see trough the texture of the side, like it was missing it or the texture only exist on the side you are.
At the image you can see that there is only texture in the outside of the door.
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Might as well add the same issue for iron doors and iron/wood trapdoors.
edit: If we're going for consistency, the icons for the old doors should be updated as well.
There also seems to be an issue with custom acacia doors, insofar as there is no _inner file, and the applied transparency seems hard-coded? Shouldn't these 'thin' blocks with transparency (doors / trapdoors) just be simple extrusions not requiring an inner face?
That wouldn't be a good idea, as all blocks are meant to be changeable via custom models. Hard-coded models have been completely removed (apart from flowerpots for some reason), so adding more in is pointless.
An extrusion of a supplied custom model...

As of 14w33a even the new doors are affected.
Could possibly mark this as working as intended, since the change from 3d to less-3D occurred knowingly and intentionally
As there's still no official words from any Mojang members, this cannot be declared neither WAI, nor as an actual bug... In my personal case, I'm hoping it's the latter, and it just got worse on the last snapshot, but time will tell...

Working as Intended, see MC-66985
Looks like the 3D textures are back in 1.8, see MC-71184
Ignore the previous comment. Looks like the reporter scammed us.
Reopened since new doors in 14w32d have inside textures