My mattress is on the floor. 
But I have framed artwork, including some antiques and real paintings by actual artists. Do things even out?
(NB: mattress is on the floor because the movers lost the clamps to the damn bed frame, not because I think it’s cool and countercultural.)
I get what you’re trying to say, but I really find displays of sci-fi stuff, especially witty ones, more interesting and insightful than a repro of some “antique samurai”. Though I may be jaded from having known too many guys in their 20s who had “Asian” decor and katanas on stands. Ugh.
Never fear. We can still buy books by the yard if we want people to think we’re better than we are.
I’m an adult and have stopped following rules. I have everything on my walls from a lovely piece of crewel work done by a family member to a sign ostensibly from a hotel in Norfolk, VA, that says “Street girls bringing sailors into hotel must pay for room in advance.” I can’t decide if it should go over the front hall table or in the bathroom downstairs. 
Marion Wormer gives good advice, albeit a little on the safe side. I do think she’s right on the subject of not hanging nudes, though. Men sometimes think a nude is just beautiful, and don’t realise how uncomfortable it makes visiting women.
What do i have on my walls? Stuff from the etnic art store. A carved wooden sculpture that always gets me compliments. A carved wooden mirror frame. A rug mounted on the wall. My husbands four guitars, which has the added benefit of keeping those off the floor and out of the way. http://www.muziekwinkelmusicas.nl/gitaarhaak-gitaarbeugel.html i use one of these brackets for easy and safe hanging of the instruments.
A mounted and framed Large piece of softboard that serves as a rotating exibition of posters and my kids drawings. In the hallway, two more of those large softboards, one with household paper stuff i don’t want to get lost, one with received greeting cards and pictures.
In the bedroom, i have an very uneven wall with a lot of tubes. I bought a couple of those cheap small trays to put on the table with candles. And i hung those in a group, with pictures in them of loved ones.
North Korean propaganda poster, Persian carpets, a wooden propeller, 5 clocks showing time around the world in cities I have lived in, a copper relief from the Republic of Georgia and an oil painting made by a friend in Prague.
So, you want people to think you’re terribly boring? Or, equally likely, that you’re deliberately hiding something? (Which, by your own admission, is true.) Your description makes me think of art selected by corporations to break up the monotony of office walls; not exactly what most people would consider a standard for sincerity.
C.S. Lewis: “When I became a man I put away childish things, including the fear of childishness and the desire to be very grown up.”
My tastes are eclectic: prints by a favored fantasy author, original sketches of webcomic strips, maps of places both real and fantastic, musical instruments, shelves populated by everything from odd devices of brass and glass to MLP figures. I hang what I please on my walls, and people may draw what conclusions they wish. If they think less of me for it, they can go hang…stuff on their own walls. They probably don’t belong in my house, anyway. (To date, no one who has visited my house has found fault with the decor, and many have expressed admiration.)
I’m not sure if this point has been made yet:
If you’re a certain type of mature adult, “What do I hang on my walls?” is not a question you consider in isolation. Rather, it’s part of the larger question of how to furnish and decorate your home—what total effect are you aiming for? (If you’re really serious, and rich enough, you might hire an interior decorator to do it for you.) So, you choose what to hang on your walls to go along with your furniture, draperies, and all the other things about the room whose walls you’re decorating.
For example, a movie poster, no matter how classy it might look, is going to be out of place in an elegant Victorian-style parlor.
So, here’s why I was so conservative in my recommendations. When you’re young and you’re starting to decorate, you don’t have much experience. If someone gives you conservative advice, you have an opportunity decorate in a conservative way, then loosen up as you gain experience and put your personal stamp on it. If you hang onto stuff from your youth too long it’s much harder to shift into adult decorating mode later on.
If you pick well, as your tastes change, you can keep stuff and move it around your home as you move into more adult relationships/home/profession. I started out with framed reproductions of aviation events from WWI and WWII. As I got older I added abstract art and landscapes and framed fabric art. The aviation posters moved into a hallway.
And for the recommendation about ‘who you want them to think you are’. Let’s say you’ve met a potential life partner. Do you want him or her to believe you’re a reliable partner? Then being a little conservative in your decorating your public spaces is part of making an impression that you’re thoughtful of others.
You can always express your interests, but doing it in a tasteful way, is part of ‘putting away childish things.’
I know there’s been disagreement with my suggestions. But I think of them as a starting point for a young person, not as an endgame for everyone.
We have mostly our own artwork on the walls. My wife works in watercolor, though she recently has started in acrylics–all the lumpy fun of oils without the six month drying time. I am a photographer, so my pictures can be found throughout the house.
Mature adults hang photos of loved ones and relatives, certificates, calendars (not Playboy though), and landscape paintings on their walls.
By room:
Computer: one wall is genealogy related photos and a family coat of arms; there is a William Givler print of Celilo Falls fishing on another wall; a large framed print of a Tlingit motif; my Taylor 414CE; a mirror.
Living: a one-off aquatint Japanese street scene; two African masks; a woodcut (artist unknown); four original George Ahgupuk pen/ink Inuit village scenes; a water color of a Russian church; a telecaster and a Brazilian acoustic.
Dining: an Alaska artist Fred Machetanz print; two old photos of Alaska; another Ahgupuk pen/ink; a large scrimshawed piece of baleen; a wine rack; three Japanese prints.
Bedroom: A Machetanz stone lithograph, some African stuff, and a series of Marie Watt prints.
It’s an eclectic mixture. Put up whatever you want.
If you want to go for maximum visual impact, consider a large format canvas print - things like landscapes or cityscapes work well here.
By large format, I mean wall sized - where I work we have a print of (I think) Singapore that’s six metres wide and it looks stunning. While it’s not overly cheap, it’s not as expensive as you might think, either - maybe £100-£200 or so, depending on size.
Or start a new trend 
But . . . Good Art Won’t Match Your Sofa
I have a trunk lid from a Porsche race car hanging on the wall. 51 years old, and I don’t give a shit if it’s mature or not.
Win.
I’m a woman and I have nudes on the walls in my living room. Granted, they are super famous cubist nudes, but nudes just the same. I also have flowers and birds and landscapes and street scenes and family photos and stuff though, so I think it balances out. If all of your art is nudes that might be a bit much.
I have this on one of my walls. But then again, I have a lot of weird stuff on my walls, and I don’t care what people think of my taste in art.
Please tell me the clock is real, but runs backwards.
My mother has a nude and a “woman with capote” (a heavily-embroidered cloak, worn by bullfighters as they enter or exit the arena), both of which are a friend’s portrait of his wife, in her living room. She got them from the wife along with several sculptures (the wife’s work).
I don’t know if Mom is a grown-up, but she’s 75.
