Descent was the first example of an FPS-style action title which divorced itself away from the sprite and raycasting approach of games prior. People often think of Quake as the first polygonal 3D FPS, but Descent predated it by a year, and more importantly, really shifted the gameplay mechanics (something Quake cannot really claim per se).
Beyond simply giving you a genuine 3D environment, it also fully exercised the 6DOF freedom of movement afforded by using a 3D environment, and escaped a lot of the limitations of games that preceded it (and even a few that followed). Which makes it both an interesting gameplay case study as well as a challenging game to play. It has stood the test of time in that it still sees mods to this day, and yet it is unique enough that it hasn't been duplicated as heavily as many others.
-Parashar Krishnamachari, Crystal Dynamics
Tribes was one of the first titles that saw the popularization of teamplay and the 'capture the flag' scenario as a critical game element. It also introduced the first seamless multiple player roles, team communications like waypoints, a unique quickchat system, indoor-outdoor rendering engine, higher player count than ever before, and unique 3-dimensional gameplay with the addition of player jetpacks. It consistently ranks in the top 40 games played online still today, almost 8 years after its release.
-Alex Swanson, GarageGames, Inc.