Wikipedia
actually has a really nice article on RTS games and their history..
But basically, it all boils down to this:
Pre-Herzog Zwei (a lot of titles!): build of strategic/economic concepts and other gameplay related mechanics.
Herzog Zwei: core concepts of an RTS game.
Dune II: mouse-driven interface; refinement of core concepts.
Total Annihilation: refinement of interface.
Starcraft: refinement of multiplayer gameplay.
Post-Starcraft: step into 3d, specialization, experimentation. Note however that not many games after Starcraft have really influenced the genre much; Strategic zoom (Supreme Commander) isn't in all games, 3d movement (Homeworld) isn't in all games, etc. But you bet your ass almost all games contain features from the core four influential titles!