I was somewhat disappointed by Half-Life 2 but I also enjoyed it a lot. My basic expectations were formed by the following three trailers that made it seem like Valve were aiming for a more prohibitive, survival-style approach to encounters with enemies, so that avoiding enemies, taking them by surprise, or making dashing escapes would be the game's thing (somewhat similar to encounters with the Marines in the first game, which could be deadly before getting used to them). Here are those trailers:
The soldiers seemed like a hazard to be avoided, the physics traps seemed elaborate, more important, and sometimes even not so clearly signalled (the container), the zombies seemed like they could swarm the player, there was a weapon-limit that would make the conservation of resources more important, and Combine's synthetic creatures seemed like a terror to be avoided at all costs. Added to all that, in order for those elements to make sense and work, the maps would have to be a little less linear. Valve revealed somewhere that for example they planned for the canals chapter to be non-linear, but they found that their testers were trying to explore everything, which Valve didn't want them to do, so they made them linear.
What the game turned out to be, as a shooter, was simply easy, bordering on triviality one third of the time perhaps. The weapons might not feel that powerful, but they're enough to obliterate the enemies most of the time as soon as you see them, so while the soldiers have adequate AI (they can flank, they do have many situational comments) they simply don't last and the player has no motivation to experiment (relying on the shot-gun and the gravity-gun more made the combat more enjoyable for me).
The game's main strengths are the set-piece encounters, the well-executed cinematic style (I mean both the integration of cutscene-like moments into the flow of the game and how cool the combat set pieces can look), and the aesthetics. I think the challenge was basically sacrificed for the sake of the flow and immersion. The set pieces make for varied situations and they're nice as arranged action scenes – more like immersive scenes than segments that are engaging in terms of the game-play. This is especially appart in the first third or so of the game, where weapons like the pistol and the SMG give audio-visual enjoyment more than anything, and even Ravenholm relies on that sort of thing.
I get the complaint about how different the various sections are, but I don't think the vehicles are a gimmick, they're pretty fun to drive, and the physics are implemented well enough to be perfectly serviceable (throwing back grenades by hand or with the g-gun is especially fun). The friendly squad was pretty useless and annoying, though.
Finally, the story is quite fitting actually if you ignore the character-centric family part (the main writer, Laidlaw, said that it was his big central idea for making people care about the story). The G-Man has some sort of a shady deal with the resistance and Gordon is not quite there for the reason the resistance people want to admit. He acts more as an agent provocateur, the uprising seems doomed to fight against impossible odds, and Gordon possibly destroys the city by blowing up the Citadel at the end (sadly disproved by the Episodes). It has a nice eerie feel to it, what with the G-Man's ulterior motives, and Breen's being kind of a villain with reasonable motivation of a collaborator. The dialgoue is also pretty well written and acted much of the time, it's just not very interesting to see all those scenes on replays.
All in all, I think Valve wanted to make the game more accessible to people, emphasizing flow and immersion as the most important factors, making it not a casual shooter, but more like shooter light. The Episodes were shorter and more tightly designed, and it was easier to enjoy them for what they were at that point.
