The first thing that struck in Berlin was how few people were on the roads. I don't think I saw more than five pedestrians in the entire trip from the airport to the hotel. There was also hardly any traffic on the roads. I don't know if it was the day or time because there were more people on the other days.
Another funny thing was the subway/metro system. You need to buy a ticket, but it is not checked anywhere. It is entirely possible to ride the subway without a ticket. In all the subways that I've seen, you have to put your ticket into a machine to be able to enter the platform, but not here. The other cool thing is that you can get a 3 or 7 day pass that not only allows unlimited travel on the subway but is also valid on the buses and trams.
The difference between east and west Berlin was also clearly evident. West Berlin was a lot more crowded during the day, but the situation was the opposite at night. The trams are mostly seen on the east Berlin side, and the building architecture is noticibly different. Oh, and of course, the pedestrian lights are different (see this).
The new Berlin station (Hauptbahnof) is really pretty (More about it here).
Another concern that I had was safety. It felt pretty safe even in the metros and the streets around midnight. Maybe the extra security for the world cup helped. Roaming the east Berlin side was nice especially around Friedrichstraße, and Unter den linden, two important roads in this part of Berlin. Kufurstendamm on the west side was also good.
Also notable was the number of cyclists. Cycling around seems to be very popular, and you can find cyclists everywhere. There is a special coach on the subway for cyclists to use. There are even some dressed in suit and tie cycling to work.