Not to be a "hater," which sounds kind of harsh...but we're not really into decorating our room. Mostly they are okay, some are even cute. A few tasteful, discreet decorations are fine.
Some 'decor' I've seen was over the top.
One example that comes to mind was not so much a window decoration as a full king sized white sheet tied to a room balcony and painted with crude green lettering. It was really ugly and IMO, ruined the look of the resort. Beyond that, the person loudly proclaimed their DIS identity, which kind of, IMO, made the DIS look a little bad. You could see this thing from QUITE a distance! All the way from the park, or pretty close to it!
Any decor that causes damage to the room, IMO, would be over the top. (tape that takes off paint, stains, etc.)
Anything that well, featured something inappropriate would be over the top - like Halloween decorations that are scary for small kids, profanity, Mickey doing something sexual. For me, that includes putting up overtly non WDW characters. I dont mean generic smily faces, I mean putting up something like a giant Scooby Doo; a big, bloody, death skull; or anything REMOTELY political. Those kind of decorations take away from the experience of other guests.
Also inappropriate would be any exterior decor at AKL that has even the slightest chance of becoming dislodged and falling into the animal pen.
Leaving something like fake spray snow all over your window for the staff to clean up is inappropriate, IMO.
Anything that's a fire hazzard or is dangerous in any way, would be inappropriate (Again, I'm thinking Halloween decor, like the kind I've seen sometimes at private homes)
I dunno. Like I said initially, if you want to put something discreet like a few small Mickey clings in your window that's fine. Probably a small hint of Christmas during Christmas week is fine (I've not been that week, so I don't know). Most of the decor I've seen is okay, but mostly I prefer to leave the decorating to the WDW staff. Disney has worked REALLY hard to create a certain look in it's resorts. Anything that clashes with that look (See tacky sheet description above), takes away from that experience for guests. A small infraction to be sure, but an infraction nonetheless. You are imposing your tastes/feelings on everyone else's vacation.
Someone, at some point, is going to create a liability for Disney.
No, I take that back. It's already been done.