Fallout 3 is a game with history. It has antecedents like Fallout 1 & 2 or Morrowind and Oblivion; it has a fully fleshed out game world; it has a full role-playing system. These are all valid, if entirely obvious, points to discuss for comparison. However, playing the game itself evokes a different kind of feeling. The slow uncovering of a game world filled with interesting nooks and crannies evokes the best aspects of the exploration-based Castlevania and Metroid games, while the role-playing system successfully melds Fallout with a first-person shooter.
Fallout 3 is built around exploration and character development more than previous games in the series, thanks primarily to its switch to a 3D engine. In previous Fallout games, each important part of the game was segmented off from the next. The player traveled to a town over a world map. This is gone in Fallout 3, replaced by a contiguous world where the player can walk from one side of the map to the other. In addition to that, it rewards the player who chooses to walk by showing interesting things to walk towards, in addition to having a compass with a marker showing where unexplored areas are. The scope is smaller - it's simply the D.C. area (now the "Capital Wasteland") instead of half of California - which is somewhat disappointing only because the game makes the player want more.
This makes Fallout 3 feel like it is unfolding naturally in front of the player – Where you are feels exactly where you're supposed to be. Certainly, some parts of the game are harder than others, but ideally, the player soon realizes this and wanders in a different direction. This is why it is such a shock when, upon finishing the main quest, the game simply ends. Sure, this happens in most every other RPG, but in Fallout 3, this sudden, arbitrary imposition of boundaries was a betrayal. Likewise, the official downloadable content released by Bethesda disappoints largely because it takes the player away from the Capital Wasteland instead of providing more to explore.
That Fallout 3 was even released, given the distance between it and its predecessors, was a pleasant surprise. That it's a great game, and a worthy continuation of the name is even better. That it takes the franchise in new directions, with new perspectives, while maintaining much of original games' charm makes it a modern classic.