Episode One delivers all the best gameplay from Half-Life 2
Ok, let me just say up front, as many others have stated, Episode One is short. But that doesn't make it lacking in game play in any way shape or form. I don't know how Valve did it, but they managed to squeeze many of the best gameplay elements into an action packed 6-7 hours. It's kind of like making Lord of the Rings into a 100 page novel.
HL2-Ep.1 takes you back into City 17 just as you remember it; In total chaos. The game play is the familiar, but graphically they have added HDR support, and the voice acting and overall script seems so much more tight. While there is nothing that groundbreaking, Valve did a great job maintaining the great atmosphere you were used to in Half-Life 2.
There are more zombies, headcrabs, Combine, antlions, and yes striders to hinder your progress along the way. The gravity gun is critical in the beginning of the game too. But the most important aspect that stands out is the cooperation between yourself and Alyx to make your way through the story.
There are several puzzles to solve along the way, and some pathfinding to get to where you need to go. While creative, a couple of the puzzles are repeated too many times that it gets long in the tooth real quick. And finding one's way at times can be a little confusing. But to no avail, it only mildly degrades from the overall enjoyment of the game.
Overall, for an expansion, Half-Life 2 Episode One is top notch. It is a great continuation of the original story, and explains where things left off after the original one, albeit a little bit trite. Highly recommend to any fan of the original HL2. And if you've never played Half-Life 2, then why haven't you? :) While HL2 isn't a prerequisite, you will be thoroughly confused with Episode One.
Good Luck and am looking forward to Episode Two.