The Potsdamer Platz Arkaden has most of the main retail stores that can be found throughout Germany, but my favourites are the H&M store with all the great affordable clothes for men, women and kids and the H&M Details store, which is gorgeous and has great, sexy lingerie at really great prices! After 3 months in Europe (and checking out all the shops along the way!) H&M has become my favourite shop, especially the Details store in Berlin. They have great clothes, and the prices are great too - I found an adorable little red tweed coat for only 70 euros (would have paid way more at another store) and a gorgeous underwear set for only 25 euros.
Definitely worth a couple of hours (especially the Details lingerie store - spent several hours there trying everything on - great changing rooms too!)