Just got off Disney Magic Cruise where we could decorate our doors and hang a mailbox (Fish Extender) from our Fish. Do the resorts have magnetic doors or fish hangers too?
You likely won't see the fish extenders, but if you are staying value or mod (with the exception of the suites at AoA), you will see a lot of decorated windows- window clings.signs, holiday decorations, stuffed animals, etc. some families go all out, but usually it's just a few cute things. The Mousekeepers will sometimes arrange your stuffed animals in the window in a curtain hammock, too.
I have no idea what a fish extender is -- but not all of the resort doors are steel. I think it depends on whether the door opens onto an interior hallway or an exterior hallway. For example, I remember checking at Boardwalk Villas, and the door was wood - a magnet definitely would not stick.
The FEs work because a cruise starts & ends on the same dates for everyone on board. A hotel is not so simple. People are always coming & going on different dates. You might be able to connect with other families that will be there the same dates as you but unless they've cruised DCL, they likely won't "get it" nor feel the magic that you did.
Also I think people would be more reluctant to have gifts outside someone's door since there's so much more traffic. The odds of something being stolen is much higher.
We saw a family put up a dry erase board with a different poll each day - What is your favorite park? Who is your favorite character? etc. It was neat to see all the votes. We passed this room every day and had alot of fun voting!