What you can expect in Eastern Europe, is first of all unspoiled destinations. You will have the opportunity to avoid the mass tourism now that it is still possible and visit many cities with a large possibility of sightseeing opportunities, in some cases as much as in any other touristic destination. The word to describe very generally Eastern Europe is "contrast".
Contrast means basically that you will find there cities that try to be but still aren't. You will find commercial centers and high skyscrapers in the city center, surrounded at just some 100 meters by buildings that are on the way to fall down, sometimes even placed wall to wall. The highly demanding society that belonging to Europe means to be, compared with the actual resources that this countries have, puts them in a bit of trouble when it comes to select the adequate investments for the growing cities.
Some of those countries suffered a lot during the recent world war, specially Travel Guide all the ones that used to belong to the old Russia. All this countries where placed in the middle of the way, and German troops and allied countries created an absolute chaos on the territory. Tallin, for example, had more than a 60% of its residential areas destroyed by bombings.