Half Life 2 Episode 1 was much like the Opposing Force expansion to the original Half Life. Both games were largely similar, but each offered a distinct twist that helped it along. In Opposing Force, it was the concept of playing as a soldier hunting down Gordon Freeman, and the benefits of having a troop of specialized AI soldiers to help you along the way.
In Episode 1, it was the impressive AI of Alyx Vance, which helped you bond with her character as well as giving Valve a chance to create some interesting scenarios for her and Gordon to tackle. Both games also benefited from their incredible set pieces that improved upon most of the things we experienced in the original games. It says a lot when your two expansion packs contain some of the best single player content of the year. → Readout 3: Takedown