Friendly warning: this post is full of spoilers and intended for those who have finished Half-Life 2, Episode One and Episode Two.
Episode One occupies this strange transitory space between Half-Life 2's broad narrative strokes and Episode Two's relatively heavy-handed exposition. There is a great deal of foreshadowing of events to come: the transmission from Judith, the emphasis on Eli and Alyx's strong father-daughter relationship, establishing the Vortigaunts and the G-Man as key players in the greater conflict, and of course bringing the dreaded Advisors creeping into the limelight. Yet much of this foreshadowing can go unnoticed on your first play-through as we know very little about the importance of these elements until Episode Two, so your attention remains focused on the pressing urgency of stabilising the Citadel core and escaping City 17.
Given that Episode One largely takes place in environments familiar to players of Half-Life 2 it comes as no surprise that many felt it was too derivative of its predecessor. We return to the Citadel interior and fight through war-torn city streets much as we did in Half-Life 2, albeit with a far greater emphasis on indirect combat through use of the Gravity Gun, various inventive gadgets/weapons, and your trusty sidekick. Indeed, the vast majority of its art and sound assets are recycled and it features only two new enemies, one of which was present but posed no threat in Half-Life 2 (the Stalker) and the other being an amalgamation of two other enemies (the Zombine).
But one point that Episode One really nails is the sense of a world in motion (the storysense as Tadhg Kelly would put it). From the very beginning, we bear witness to world-changing events that affect the lives of every individual in the vicinity, be they fleeing citizens or desperate Overwatch soldiers. This is an area in which Half-Life games have always excelled in my opinion, ever since the series opened with the Resonance Cascade which still hangs heavy on Eli's conscience. In this particular episode we witness the changes brought about by our own actions as the Citadel reels from a heavy blow, eventually leading into a cataclysmic eruption that opens the gateway to vast legions of alien invaders.
At the end of the day Episode One functions as a great segue from the monolithic release of Half-Life 2 into the bite-size episodic format. It has some obligations to tie up certain loose ends (the fate of Gordon, Alyx and the people of City 17) which flow into subsequent events (the opening of the Combine superportal and the hidden transmission of the Borealis data packet) but it places such a firm focus on the immediate goals that I for one was swept up in the adventure before stopping to consider how it all fit into the bigger picture. I believe it is for this reason that replaying this - or any Half-Life game - remains such a joy even more than 5 years later.