Tucking into a plate of Swedish meatballs, a salmon wrap or something else off of IKEA's seasonally changing menu is just another part of the experience. Roughly 700 million people ate in an IKEA restaurant in 2013, and some of those customers helped consume 150 million meatballs in the process [source: Hansegard].
In truth, the "restaurant" is more of a cafeteria, which means all of the same conventions apply. While you're in line, scan the menu and be ready to order when you get to the counter. Show good hygiene by keeping your fingers off every single salad and piece of cake, with the exception of the one you plan to eat. And if you're waiting for friends, don't ask them to join you in line when they arrive, in effect cutting in front of all the other hangry customers; instead, wait for them or have them join you at the table. There's no need to add to the commotion by shouting, and, if you're visiting with squirmy children, ensure they remain seated. Shockingly for some, the cafeteria doesn't double as a playground.
When you're finished, don't be the person who conveniently "forgets" to bus the dishes; take your tray to the proper cart and wipe your mess off the table. And since no one wants to sit on a coffee-stained couch or snuggle up on a crumb-covered pillow, restrict all of your calorie consumption to the cafeteria.