After finally playing it, I wasn't too enthusiastic with Half-Life 2. It does a lot of things right (voice acting, facial animations, physics, level design, art direction) but it felt like Valve had some great ideas and just stretched things out too long and reused scenarios too much.
1) The vehicles. Driving the Airboat was fun at first but it goes on and on far too long until it just gets boring. Same thing with the dune buggy. And the vehicles weren't seamlessly integrated into the levels either - you get one boat level and one car level.
2) Setting up the turrets to defend that one room was cool at first. And then Valve repeats the very same thing just like two minutes later. And then a THIRD time right after that!
3) Fighting the Striders. After the whole game of seeing them you finally get to take one down and it's satisfying. The first time. But fighting like six or seven more all at once just felt like overload and it's tedious. That was probably the most boring chapter of the game for me.
4) The loading.... for all the thought Valve put into the level designs to make them seem more realistic and part of one big world, the dozens of loading screens completely ruin that immersion. The worst part about it is that they're in shitty spots - like one near the end of the Airboat level: you're making a huge jump and thinking "woohoo" and anticipating hitting the water in a satisfying splash... But before you even land at the bottom you hit a fucking loading screen which totally ruins the moment.
5) The gunplay just feels really dated when compared to other FPS such as FEAR and Halo. You can't melee nor could you throw grenades in the middle of combat without switching to them first. The AI isn't too great - Combine seem to mostly stand around in place in front of some exploding barrel or something, zombies move extremely slow, and the manhacks and headcrabs aren't any fun to fight against, they are just annoyances.
6) The NPC's. The collision detection sucks, they just got in the way far too much. When the antlions were following me and I'd explore a cell or something they'd always block the doorways or get in my way. Same for the human NPC's, and I know Valve realized this cause they coded something into the game where they say "Sorry [for getting in the way]" after blocking the door for five seconds. Thankfully this was fixed in episode 1. And the other thing that annoyed me about the NPC's (although this is minor) is the constant praise. Everyone was always saying crap like "Oh Gordon, how did we ever survive without you" and it got irritating after awhile.
I liked the episodes MUCH better. They felt a lot more polished and had far less filler.
Really loved episode 1. Valve did a lot more with less IMO. There was just one gunship fight (in the wooden attic) but it was actually memorable and fun. There was one Strider fight but it was really great. The set-pieces for those two fights were amazing. Starting off with the super gravity gun was fun, and the pitch black tunnel where you're actually forced to use the flashlight was pretty cool too. The pacing was much better in this game.
Best of all was Alyx, probably the best NPC/AI I've ever seen in a video game. I've never seen her do anything stupid and she didn't need much help - Alyx actually helps YOU. I shined my flashlight on the enemies and she started taking them all out. The part where she covers you with the sniper rifle was good too. I liked that she was very chatty too, it made her character feel much more real.
Episode 2 was also great, but aside from the environments and two new enemies it felt like there was barely any new gameplay moments in it. Almost everything felt like it was lifted directly from HL2 or Ep1. The driving parts were exactly like HL2 (even the helicopter dropping bombs in front of you, except now you're in a car instead of an airboat). The "seesaw" puzzle was also repeated (with the bridge), defending an area with setting up turrets (though it was MUCH better here), getting across a hazardous waste area by jumping on shit for like the 500th time, the strider encounter at the end where you're facing several of them in a row like the first game, it just all felt very familiar. The ending was well done but extremely predictable.