Faster and more intense than Half-Life 2.
Half-Life 2: Episode One is Half-Life 2, distilled. That is, it is a much more refined and intense experience. If the movie analogy works for HL2, then Episode One could best be compared to a ride on a roller-coaster: the entire time spent playing is exhilarating, barely-controlled chaos.
Everyone who played HL2 has memories of the game's epic moments. The confusion of the G-Man's opening monologue, the unarmed flight from the Combine over the rooftops of City 17, the first moment that a fellow resistance fighter shouted "Strider!" and the bone-chilling roar as the titanic creature crashed into view. Episode One is nearly entirely composed of those moments.
The gameplay is heavily action-oriented, with some fairly challenging puzzles thrown in for good measure. The HL2 arsenal is available in Episode One entirely unchanged. The only notable addition, is Alyx's side-arm that she uses through most of the game, and unfortunately can't be used by the player. What this says about the HL2 weaponry is that it was and still is exceptionally well-rounded, and the addition of another gun might have seemed unnecessary.
As for enemies, the most noticeable inclusion to Episode One are the Zombines, or Combine forces turned into zombies by the headcrab's corrupting embrace. Not only are the Zombines tougher than their less armored counterparts, but they also have the most annoying habit of charging madly forward with an armed grenade clutched in their hands. Fortunately, it's nothing the gravity gun can't solve.
Episode One's graphics are more or less the same as HL2. That is to say, some of the best to be found in a video game. The facial animation system seems to have been refined, as well some of the shaders and textures in the game. The carapaces of the ant-lions and the headcrab's hides now have a very alluring shine to them, which should be admired only when the beasts have stopped moving... The environments of Episode One are just as gorgeous as those of HL2, if somewhat more enclosed-feeling due to the linearity of the level design.
The voice acting of Episode One is also superb. Alyx receives most of the dialogue in the game, resulting in a surprising amount of development of her character. Despite the fact that in HL2 Alyx was more of a bystander to the main story, in Episode One she is at Gordon's side constantly, providing some genuinely funny comments that ease the tension of the game. The music in Episode One is subtle, as it was in HL2, but seems even less a part of the entire experience, with fewer crescendos noting the major moments of the story.
The debate surrounding the value of episodic games in general (and Episode One in particular) is fairly heated. Some say that it cannot be taken into account in the same perspective that it is applied to "full-length" games. But unfortunately, it has to be done.
Before any episodic games had been released, I had concerns about the amount of gameplay that the player could hope to get out of an episodic title. But I was pleased to say that SiN Episodes: Emergence changed my mind. For the $18 price, I was able to enjoy the game for around 7 hours on my first time through. Half-Life 2: Episode One on the other hand, I finished in less than five hours.
The length of the game is largely due to the very fast pacing. Intensity is something that is hard to sustain, particularly in a game, without the experience becoming mundane or overly difficult. And that fact is really what makes Half-Life 2: Episode One all the more impressive. Valve has attempted to provide a driven, cinematic story while still offering a fun gameplay experience. And in both of those attempts, they have succeeded.