The half life of Half-Life turned out to be about seven years
It works. Doesn't have any bugs. Very polished. Few memorable levels. Difficulty is just right. Lasts little longer than other FPSes.
The problem with Half-Life 2 is that it's just "a" FPS. It lacks something - anything - to elevate it from being just a game and make it a truly great.
Setting was a definite strong point of the first game; here, it's almost nonexistent. The lauded Orwellian atmosphere lasts up to the point when we grab the first gun, which is about fifteen minutes in. Then it's you versus the world in usually bland and boring linear levels. Not only was the first game less linear, when it was it made a nice job of hiding it; Half-Life 2 is completely linear and fails utterly at hiding it. Both games are very lean in story department(and yes, I do read that site listing all references and speculations about stuff like graffiti on walls and whatnot but that's what they are - speculations), so setting is extremely important. But it's just nowhere here nor there - the world seems more like wasteland than occupation. Ties and references to Half-Life seem forced.
The chapter structure of the game definitely doesn't help. The chapters can be vastly different from each other and the changes feel awkward. "Now you drive a boat" "Now you are scared" "Now you drive a car" and so on. Half-Life too had different sections, but you moved between them smoothly and meaningfully(usually by achieving a goal which was clear from the beginning), not just because the developers decided "Okay, that's enough, next".
Even the weapons and enemies compare unfavorably. The weapons are especially irritating because there is no "the" definitive weapon in the game. The SMG is just a little too underpowered and inaccurate, and the ammo for the Plasma Rifle is just little too sparse. Plus the weapons in general are quite uninteresting - gone are exotic and alien designs of Half-Life, instead we get bizarre stuff at best - like a crossbow firing red-hot steel bars. Couldn't it just fire normal steel bars? Couldn't it just be a normal crossbow? Well I guess then they couldn't put in pining enemies to walls - gimmick.
Actually the same story is with Gravity Gun. I mean it's not like anyone will go into a firefight with it - bar the challenge of finishing the game using nothing else or whatnot. So other than the obligatory section where the player is literally forced to use it, its only application is solving banal puzzles. This is the iconic weapon of the game, mind - like Quake 2's Railgun - and serves nothing but gimmickry.
Enemies are same case as guns, and you'll spend majority of the game fighting soldiers. Some of them wear body armor, some of them are more blue, and some are white, but the generic-faceless-human-enemy archetype persists. And no, the AI is nothing special. In fact, the AI in Half-Life seemed better, even though it was mostly just running in random directions.
And the fighting itself is standard FPS fare: You and enemy shoot each other and you win because you have more HP and do more damage. Then you restore your health and ammo using the omnipresent crates. That is the most primitive way of making combat in FPSes work; it's actually below the level of Doom - where many enemies had attacks which could be avoided by strafing and melee enemies could be outrun. In Half-Life 2 it doesn't matter if you strafe, crouch, use corners for cover or jump around - you will get hit anyway, and if the game didn't throw all these medkits at you it would be nearly impossible to finish. And that's just bad design.
The game tries to be very verbose and make the player feel sympathetic towards the many NPCs that help us as we slaughter everything. However, it fails utterly for a very simple reason: Gordon doesn't say anything. Ever. Even when people are talking directly to him, even when they're discussion matters of grave importance, Gordon acts as if he was but a hardcore mercenary who doesn't give a crap about anything and only wants to know in which direction he should slaughter next. Apparently this is intended and was supposed to make the game feel more personal - how? Am I supposed to speak instead of Gordon? You expect me, sitting alone in my room, to talk to the monitor? That's not immersion, that's being crazy.
Sure, Gordon was a mute in Half-Life as well, but the dramatic difference is that NPCs in Half-Life were clearly task-givers; you approached them, they said their script(about two lines), and then turned into docile sheep who could follow you or "wait here". Half-Life 2 introduces over ten minute long cutscenes packed full of social interactions, and leaving the player out of them is just... well, the bottom line is, what was supposed to be immersion-enhancing feature only ended up breaking the immersion pretty much every time someone speaks.
Even the graphics aren't all that great. Doom 3 is all around superior in that department, and the low polycount of some levels is glaring(for example fronts of many houses are completely flat, and the windows are simply part of the texture). This is probably because of the long development time and more of a side note, as the flaws are not really noticeable unless you're actively looking for them.
The physics system is there, but other than few puzzles the game doesn't actually do anything with it, so it's barely worth mentioning.
The Bottom Line
It's not a bad game. It's just barely above average. Even if you're resistant to some of its flaws, it certainly is not a revolution, visionary or "raising the bar" as the developers themselves audaciously put it.