Put backpacks or boxes in those 2 slots to open up even more available space. In fact, two 24-slot boxes can make...well...a lot of storage. Those slots are also used when you put items up for sale.
Just make sure you purchase a bulletin board first, avaiable from many vendors, and mount it in your room. That will allow you to sell items in your personal (body) inventory while in your room, and the items in your room vault when you are out.
Those vault items will remain for sale up to 24 hours after you log off. Once you log back in, the timer is reset.
Always nice to enter the room and hear a little cha-ching from a sale while I was away. :)
No real benefit I know of for the wallpaper, but there are many folks that enjoy decking out their rooms with all kinds of items, and calling it home. Makes for a little better experience than always going to sit in a corner of the city to "go to sleep".
There are also pets you can either buy or obtain through quests that can live in your room. I have two little boars with flaming eyes that wander around my room. Got them from the group arena in Splitpaw.
You can buy better rooms for more money and status points, but they are more expensive to upkeep. A guildmate of mine has a 2-bedroom place in Qeynos Harbor with extra beds, furniture, rugs, art, a desk with all his collection quest awards, and his Heritage Quest reward items are mounted neatly on another wall. I sometimes use his house as a log-off spot. No real benefit, just for fun.
A while ago our guild held a meeting at another 2-room house. The owner made sure it was clean, the pets were fed, and our Provisioner brought food and drinks. It was hilarious to watch everyone ham it up for the guild screen-shot. The Ratonga made quite an effort to leap on top of the wall sconce, and I got busted for jumping on the bed.
Lastly, 5s seems like a lot now, but over time you will make 10 times that from loot on with a single kill. Until then, you can do several quests to get items for your room.
Have fun!