Stockholm people are often in a hurry, trying to be efficient with their time. This is why you find everyone standing to the right in escalators, leaving the left side to those who want to WALK up. This is taken quite seriously and at rush hour, you may even find yourself being shouted at if you don't respect this rule and stand in the way of people.
Well, being 21 I thought I would be able to get in anywhere as long as I'm properly dressed. In Stockholm, quite a different story! Age limits at nightclubs are pretty high, usually around 23 for guys, sometimes even 25 unless you know the doorman. As a tourist, you better make sure you're dressed really well and have an ID on you which says you're at least 23, otherwise, I would suggest staying near the pubs at Slussen!
At most places with customer service (e.g. Post Office, Tourist Information, Money Exchange) you have to take a number from a machine. Then you have to wait for your number to be called or to be shown on the display with the appropriate counter. So instead of waiting in a line you are able to browse through other things while waiting for your number to be called.
Almost everywhere you come you have to pick a number, then you have to wait until your number is on the display before it's your turn. This system is used in a lot of places and not only the post-office building. So when you enter a shop or another establishment first look around to see if you have to get a number. If you forget to pick one, you might find yourself in an argument with some swedes! ;-)