I completed HL2 two weeks ago at a friend’s computer in a single weekend.
The levels are very nicely/realistically connected & actually give out a nice illusion of continuity in the story line flow. I haven’t witnessed this since I completed Unreal 1 back in 1999! The enemy AI was great & those troopers with the blue-eyed mask/helmets were the most sinister looking foes I’ve seen in a FPS for some time now.

My favorite highlights were:
1. Overall the physics engine in HL2 was great. Probably even better than the all mighty Doom3!
2. Incredible outdoor scenes. The city surrounding the Citadel & the sea view next to the road on the cliffs was truly breath taking. (Only Far Cry can beat it in this department)
3. The part with the hovercraft, where you race through the polluted river in the outskirts of the city. I loved the part where one of the rocket tanks blows down a tall chimney and you speed right under it just in the nick of time. Later on when you get the machine-gun on the hovercraft you 1st encounter those cool 4-engine spaceships flying over you & dropping mines in front of you.
4. The transport spaceships with the 4 engines & the gunships were the coolest looking air-born vehicles I’ve seen in a FPS. They definitely had a Japanese/Anime feel into them. I loved fighting them with the laser guided rocket launcher. Specifically, the gunships looked like Mecha/Squids!
5. The part where you 1st get the buggy and drive through a very long route, while having to stop and use the huge crane to drop down the bridge & fighting the ant-lions.
6. I thought the part where you get trained by the DOG-Mech, to using the gravity-gun, was a very nice touch. Later on when hell brakes loose and DOG opens the huge gate that lead to the zombie city and then slamming it in your face, gives you a shiver down the spine that you’re completely disserted, compared to the warm home-feel of the training.
7. The Gun Fight with your comrades at the light house after you park the buggy. When the carrier spaceship lands on top of the light and the troopers start poring out, was one of the coolest fights of the game.
8. Having to pass under the bridge with the strong side-winds pushing you from every direction.
9. The small force field protected forts, blocking the city blocks & the enemy troops firing at you from on top.
10. The street warfare at the besieged city around the citadel, where the rebels were fighting against the Walkers and you hiding under trenches as the storming Walkers were rampaging right by you, was a shocking experience indeed!
11. When you 1st enter the besieged city, (last level before the citadel) you encounter the DOG/Mech fighting a group of evil troopers. They try to smash it by driving an armored transport unit against it, but DOG picks it up and throws it right back in their face! A few seconds later DOG jumps one of the enemy transport spaceships and flies away. Surprisingly you meet him again later on when it uses all it’s might to lift a part of the expanding/moving wall of the citadel clearing the way for you. This was definitely one the coolest character I’ve encounter in a FPS.
12. The citadel it self was one the most shocking & sinister looking structures I’ve seen and so was the last level inside it.

Now what I did not like:
1. The 1st part of the game up to the point where you get the hover craft, was very boring & repetitive. I actually thought about stopping if my mate hadn’t encouraged me to play further.
2. The Grand/Administrator Evil Villain looked like Mr. Bean and was by far and away the most retarded looking bad guy I’ve seen in a game. I was very happy when I blasted him & his generator at the last stage.
3. The haunted/zombie/ghost city was very boring, repetitive tiring. Not very fun at all.
4. Same goes for the disserted Prison stage.
5. The way that the Gravity-Gun was powered at the last level, was very cheap imo. I was like, Umm... Yeah Right!
6. The Boss fights were very disappointing & the Bosses them selves very uninspiring. A FPS without some cool Bosses to back it up is... ahh…hmm.. Well…a novel.
7. The ending was total crap! Worse ending ever! Even worse than Quake 4

So HL2 was a great game, (definitely not overrated) with many nice touches, innovations, incredible graphics & sound and cool physics engine, but it’s short length, the lack of cool bosses (very important in my book) & the crapy ending, place it behind Doom 3 & Quake 4 imo.

P.S. I think Gordon Freeman was a wimp and the most retarded leading character in a FPS. It’s hard to believe that a scientist could operate a dozen high-tech assault weapons, fight through that entire holocaust & live to tell the tale in the end… Ahh.. Yeah right!
And no I’m no Halo fan.