Along with games like Ocarina of Time, Castlevania: SoN etc., Half-Life 2 is one of the best single-player classics!
The story was set 20 years after the Black Mesa incident and you're still Gorden Freeman. Sent back to the world you used to be to but this time it's been torn down a little bit more and bad guys from Xen are dominant. Personally I think the story's average but Valve did some cool things in terms of the way to unfold it. The ending sequence's very memorable but since spoiler's unwelcome so I don't intend to say more. Graphically, it even looks stunning nowadays despite no more being technically stunning as it was back then. But the powerful HDR still delivers some of the best lighting and shadow effects. Human characters' modelling is simply top-notch, with realistic and fluid animation, together creating some believeable moments.
As for gameplay, it's really a huge blast. The greatest thing on the whole is its thrilling pacing (and that's what Episode One sort of lacks). As a generally linear shooter, there are sure an amount of scripted sequences that you might encounter along the way. But there's still a chance for you to fail so they're very entertaining, such as a sequence where an oil can slid downstairs and behind it are a bunch of enemies. It really gets you everytime you bomb them away! And some intelligent set-pieces like you pulling a small cart on which there're some wooden crates that can protect you from the enemies' fire in front of you, both offered you some fun and served as a minor puzzle. Some of the other memorable moments include the one where you use gas to burn down a load of zombies etc. In other words, there being great set-pieces helped build up an epic cinematography atmosphere, while it's totally unlike CoD6's epic set-pieces where there is no real fun cuz most of them happen themselves, as if the game played itself when these epic scenes happened.
There are both indoor and outdoor levels, both of which functioned well enough to provide highly immersive moments. The indoor ones are more focused on corridor fights with covers, mixed with puzzles and path-finding, which is more like an exploration. The outdoor levels provide less but bigger puzzles, and huger enemies, along with some of the most satisfying driving. That being said, driving in HL2 is by no means a noob-thing to do though it's fast-paced and easy, because from time to time you'll get puzzles dealing with velocity and acceleration by the way. Enemies are always there as you drive your way through so you won't get bored. Sometimes you get the chance to crash them in the face with your crazy boat and it's like a "wow" fun! Sometimes you'd be chased by a helicopter and you arrived at a tower something, you'd leave your boat for a while and climbed up, shooting the helicopter down is like shooting down a boss.
In the later parts of the game, you get some co-op gameplay with CPU-controlling allies, where you get to shoot down the Striders, one of the most exciting enemy types ingame, with your rocket-launcher. And here comes one of biggest flaw - your team members' AI are somewhat dodgy. Especially when you want to avoid an attack, you may possibly find your mate stuck at the door so you have nowhere to go. But that's just a small issue for the whole playthrough, which doesn't hurt that much IMO.
The missions and level designs vary, as much as the arsenal you own. For instance there is a mission where you throw your grenades to the rooms of the building on the other side of the street, from the building which you're at, to execute those snipers. This mission seems inclined to switch the pace to a relatively slow and more careful style but puts some thrillers right into you.
Standard weapons like shotgun and magnum are pleasant to use, but it is the Gravity gun and Alien Pheromones that really see some innovation here. The idea of gravity gun is to draw an object to yourself and then toss it at the enemy, which is utilized thoroughly in a town full of zombies. And there are some great puzzles where the gravity gun is necessary. Alien Pheromones allow you to call for aliens in the later levels and order to attack your enemies, which was never seen before. Weapons like MP-7 and pulse refile both have a secondary attack mode. The former of which fires a grenade that explodes on impact as the secondary fire mode that is effective against clustered hostiles. The secondary mode of pulse rifle on the other hand, allows firing an energy ball (exactly those balls in Portal) whose lifespan would finally end after several crashes on the wall or something else like that. But when it's there, everything (despite overpowered boss-like ones) it touches dies immediately. If you use it in a closed and small space full of enemies, they'll all be burned to death right away.
Despite the relatively bad AI your team mates have, Half-Life 2 is a true classic that any FPS fan shouldn't miss. I'd recommend it to those who generally don't enjoy FPS too!