No Step Back is entirely focused on the Eastern Front: it overhauls the USSR and Poland, and adds the three Baltic states. It's the expansion that has most changed how land divisions are designed since its release.
Nations and Content
The USSR and Poland get completely overhauled focus trees, with far more depth than the base game version. Estonia, Latvia and Lithuania go from set dressing to having their own playable content.
New Mechanics
It introduces the officer corps (a command-point system split between generals and army bonuses), army spirits (national traits that reflect each country's military culture), a modular tank designer similar to the ones for ships and planes, and railway guns as a new heavy artillery piece.
Is It Worth Buying?
If you're going to play the European theater — whether with the USSR, Germany, or any other power that ends up on the Eastern Front — the officer corps and the tank designer show up in almost any long game. It's one of the expansions with the most cross-cutting mechanics in the whole catalog, not just country content.