The city of monument after monument. There are few cities that can compare in terms of modern historical significance. Walking down Unter den Linden toward Tiergarten from Mitte lets you hit site after site until you walk directly to the Brandenburg gate.
The true draw for me was the museums. From design to technology to cultural history, this city really knows how to celebrate history. You haven't done Berlin if you haven't done the museums.
Design History and Heaven
The bauhaus archiv is one of the singular most fantastic museums I have ever been into. Unfortunately, they did not allow any pictures inside. However, the bauhaus movement speaks to my design sensibility so strongly that this was heaven. It is not a very large museum, very unassuming. The building itself is reminiscent of the Carpenter center at Harvard in Cambridge, by Le Corbusier. Just gives you a taste of the style.
Learning from Mistakes
Some of the most moving exhibits describe a viewpoint of history that is gruesome and worthy of the deepest skeletons in Germany's closet. The straightforward and honest portrayal should be commended. I almost skipped the Topography of Terror because I thought the name was a bit too dramatic, but it was one of the best spots in Berlin. History buffs, you've found your city.