The Imperial forces exhibit an extreme degree of standardization. Barring different heights, each stormtrooper or TIE pilot looks exactly like any other. While the Rebel forces wear uniforms too, they're not so restrictive that the individual's identity is obscured. (This difference between the two factions is shown most clearly in the battles that bookend the movie, the boarding of the Tantive IV and the Battle of Yavin.)
The construction of the Death Star is completed during the course of the movie, so it couldn't be any newer. The equipment that the Rebels use, however, shows its age, especially the Y-wings.
The groups' respective headquarters also differ in that the Death Star is artificial and somewhat isolated in space while the Rebel base on Yavin IV is constructed primarily of stone (a natural material) and located in the midst of a jungle.
Even in their appearances, then, the Empire and the Rebel Alliance oppose each other.
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In the scene where Obi-Wan gives Anakin's lightsaber to Luke, he also briefly comments on the Jedi: "For over a thousand generations, the Jedi Knights were the guardians of peace and justice in the old Republic, before the dark times, before the Empire." Luke de-activates the lightsaber at the end of Obi-Wan's remark, and in a small way, this extinguishing mirrors the Jedi's near extinction.