I disagree with a lot of things said about HL2 in this thread. Like I've said, I don't think it's any more story-based than any other shooter released around the same time, quite the opposite actually. The "story" it has is very primitive, serving basically no other purpose than giving you a reason for killing stuff. Almost all shooters from between 2000 and 2004 have a far bigger emphasis on story, military shooters perhaps excluded. HL2 had a cool setting, but I don't think there was any particular story that Valve tried to tell. It might be bad, it might be good, but to me it was mostly irrelevant.
While I can understand the complaints about the lengthy "interactive cutscenes", I also think the people saying that they ruined the game for them or something are complete ADD retards. The game has maybe three "cutscenes" that are longer than a minute: Kleiner's lab, Black Mesa East and the Citadel. There are games with longer loading times than those cutscenes. It is bad and heavy-handed storytelling, no doubt about it, especially compared to HL1, and the game wouldn't have needed that kind of stuff. Still, it's pretty much a non-issue.
The game has its fair share of gimmicks, but they're in general very well made, whether it's the saw blade massacre of Ravenholm or the Antlion massacre of Nova Prospekt. As far as the amount of variety goes, HL2 doesn't have many challengers in the land of corridor shooters. The actual gameplay itself is mostly unchanged from HL1: you can crouch, jump and shoot, that's it. It's a very simple and working control scheme, and the game encourages fast-paced gameplay rather than crouching behind a box or something. Too bad the game is too easy, the hardest setting feeling like medium at best. HL2's AI wasn't as ruthless as the AI in HL1, but then again, HL1's AI relied much more on heavy scripting, and some encounters played out exactly the same every time.
I also enjoy the game's weaponry. The pistol sucks, and the SMG isn't that great either (except when using the grenade launcher), but they're the weakest weapons in the game anyway. The rest? The revolver is ridiculously enjoyable, although slow. The shotgun is great, especially with the secondary fire mode which deals double damage for double fun. The crossbow kills on one hit and lets you nail people on the walls, which is always a plus. The Combine assault rifle is a bit half-assed as far as the looks and the reloading animation go, but it's accurate and effective, sounds great and has a hilarious secondary fire mode, meaning that it more than serves its purpose. The RPG is maybe too much of a situational weapon as it is (meant for killing gunships and striders, not much else), but being able to guide your own missiles still makes it fun and lets you kill people with creative ways. In general all of the weapons have their weaknesses and feel different to each other, and the balance between different weapons is really good. Having a couple of more guns, maybe some exotic ones wouldn't have hurt, but then again, there are never enough guns.
I don't really get the hate on the Gravity Gun. Okay, HL2 shows off its physics a bit too much, and the Gravity Gun can easily come off as a gimmick. Still, it's a genuinely fun and creative weapon, and being able to use almost every object in the game world as a weapon or a shield is definite incline, no matter what way you're looking at it. The only part where the Gravity Gun kind of got annoying for me was Episode One, which forced you to use it for a pretty long while before giving you your usual arsenal of weapons. In HL2 it is mostly just a weapon or a tool among others, apart from the very last hour of the game or so.
One thing HL2 suffers from is overbalancing the game by greatly limiting the amount of stuff you can carry. Only around 20 bullets for the revolver? Three clips for the assault rifle? Three rockets for the RPG when your average gunship takes four or five? As a result you don't really have the need or the ability to manage your ammo, because the game so thoroughly dictates what you can use at any particular time. I would've preferred it if the game gave you ammo more sparingly but allowed you to carry a bit more at a time, making resource management more important.
I often hate vehicle sections in shooters, but I don't really find that much to complain about HL2's boat and buggy chapters. The shooting still mostly happens on foot, the vehicles are just there to allow for faster travelling between A and B. I see no problems with that.
I think one thing HL2 lacks in comparison to HL1 are big set-pieces. HL1 has numerous parts where you face some obstacle (usually a big monster or a piece of machinery) and need to achieve multiple steps in order to proceed: flip a switch there, fix that valve there, turn on something at the other end of the level and so on, until you can push the big red button and make something cool happen. Mostly it just meant that instead of walking a single corridor, you had to go to the end of two or three different ones and then come back, but it at least got you to turn your brain on for a while and explore a bit. HL2 doesn't really have anything like that, at least on the same scale. It stays as a linear corridor throughout the entire game.
The Black Mesa facility also offered a more plausible reason for corridor-like level design. In HL2 you're usually out in the open, and it gets pretty old to see your progress once again come to a halt because of a locked wooden door. Giving you an alternative route every now and then definitely wouldn't have hurt. Still, I can't say it'd have bothered me too much when corridor-like levels had become the norm in shooters, except that in many games it was worse with scripted scenes breaking the flow of the action much more often. Both Half-Life games managed to use scripted events with a good taste in my opinion.
All in all, HL1 was probably more atmospheric, impressive and cohesive, and in my opinion it's also a great example of computer game storytelling by so effectively using the strengths of the medium instead of using big exposition dumps. HL2 tried to preserve some things about its predecessor that didn't really work as well in the different type of setting they went for. Still, it was one of the last really enjoyable shooters I've played.