Considering this was released 2 years after Half Life 2, there is very little improvement and new content.
One of Episode 1's improvements is the graphics. There is more object detail than in the previous game and it shows that the Source Engine is not very limited. There is also a new foe introduced in this game whom Alyx names a 'Zombine' (Combine troops affected by the head crabs) and they are fun to fight because they plan to do kamikaze by running at you with a grenade in their hand so steal the grenade with the gravity gun to change all that. A new kind of "strategy" in this game is using your gravity gun to shift cars onto ant-holes to prevent ant-lions from emerging. What else new is there?
Well I am sorry to say that that is about it. The game is everything Half Life 2 was, but without the number of puzzles, immersive story, new environments, and no new weapons/gadgets (you do not even get anything new to replace the bug-bait). It not only totally recycles the gameplay from the original game, but it even recycles the same kind of environment. Each level in Half Life 2 featured environment much different from the previous level. Not only is it not like that in Episode 1, but the environment is very much like in Half-Life 2. How stupid is that? If a game is going to have the same kind of environment, or at least a small vairety throughout, I don't want it's style to seem recycled. Sure, some people may believe that the wonderful gameplay from HL2 should remain unchanged, but I'm sure there were many people that said the same thing about HL1. Hearing that, look at all the improvements made a sequel which everyone loves, so why couldn't Episode 1 add more content?
It's not only these reasons that make this game a big disappointment, but also the game's storyline. Episode 1 is not meant to be a full sequel, but that does not mean it has to be even shorter than a COD game. The game's storyline also leaves a lot to be desired. The dialogue remains good as always, and you still have the same great characters from the original game like Eli Vance's beautiful, likable daughter Alyx (who even mimics a zombie at one point of the game and had me fooled). Unfortunately the storyline is the same throughout and there is not even one twist in the plot which really had me annoyed and bored.
Although Episode 1's final level is fairly open-ended (when you're dragging squads to safety), the game is still too familiar for me to be that interested in it. None of this means that it is not great fun, and many missions were fun like the road tunnels which were very dark and creepy, but a little bit more in this package certainly would not hurt. If you still prefer the same old brilliant Half-Life 2 design, then by all means feel free to give it a shot. However, if you are expecting anything new, I suggest you wait till it's cheap or get it with the Orange Box just so you can continue the plot and understand the events in Episode 2 (which is much more improved).