The gamelength is only a few hours at most, but like the original, it's very immense.
The gameplay is the same as it is in the original Half Life 2. There will be places to crouch through, others to sprint and jump, and, of course, plenty of otherworld creatures and Combine to fill with lead. The early part of the game has you only using the Gravity Gun, but you'll be reunited with the rest of the weapons soon enough.
The graphics are about the same, if not more enhanced. Alyx gives plenty of facial expression while she speaks, but you don't see much of Judith, Dr. Kleiner, or Alyx's father. You do get to see some new otherworldly creatures, but nothing that needs killing other than the typical Combine, Headcrabs, Zombies, and Antlions.
The sounds are pretty much carryovers from the original, but there are some new BGM's to listen to while you're fighting. The new audio for the sniper rifle is nice too, and the Combine snipers are not afraid to go for your head.
Otherwise, the $20 value for the game doesn't quite go with the few hours of gameplay. The new modes of multiplayer plus the new Source version of the Half Life 1 Deathmatch mode does offset the short SP gameplay. You'll still find some of them playing the DM games, but since all Valve games launch through Steam, it, of course, pales with all the Counter-Strike and Day of Defeat players. Other than that, Episode 1 is a good standalone pack with all the MP amenities that come with it for a few hours session.