So I've played through the game twice now, once on normal mode, and the second time on hard mode carrying some dumb ass gnome. I'll never have that experience to play through the game again for the first time, and going through it the second time I started to notice some problems.
Let me preface the gripes I have with a solid recommendation: it's really really good.

The main gripe I have with the game is after you've mastered the gameplay, the game becomes easy. Really really easy. Let me give a breakdown of each main enemy and I'll explain.

Barnacles: You just shoot them. If they are ever in a spot you can't shoot them there's usually an item nearby to force grab. Most of the time you aren't just teleporting/moving around willy nilly so you'll basically never accidentally run into them.

Headcrabs: They take a bit of time to jump on you, and once they do you can simply move out of the way or block them with a grabbed object.

Poison Headcrabs: While they are more tanky than their plain brethren, they still have the main issue where they take too long to attack and can be dodged. The poison effect they give goes away very quickly as well and from what I can tell don't drop your HP to 1 like in Half-Life 2.

Armored Headcrabs: These guys are more interesting to deal with when they are on a zombie, as they will need more shots to take down, but once they are on the ground and you have either a. good aim b. a laser sight upgrade or c. decent reflexes, they die in one shot from the pistol, even on hard.

Zombies: They move slowly enough that in most cases you can simply walk away.

Reviver: This enemies mechanics are awesome, but once it's in a zombie the projectile the zombie creates is so slow it's almost pitiful, and the entire encounter boils down to "don't go near it, wait for the electric pulses."

Antlions: Super intimidating but will rarely ever hit you since they move slowly, won't jump at you like in Half-Life 2, and attack really slowly.

Projectile Antlion: This is a cool enemy, but every time you fight them there's a convenient place to hide from their attack.

Manhack: Due to the excellent sound design and the fact that shotguns exist, these guys will probably never sneak up on you and die even on hard with one shotgun blast.

Combine: These guys aren't pushovers, which in my opinion makes them the best enemy types in the game. The heavies with their giant footsteps, shield, and overall tankiness are a blast to deal with. The minigun combine are a bit bland, as once you hear them rev (thanks sound design, we don't deserve you), it's simply a matter of waiting for them to stop, then peeking out and shooting them. The AI overall for the combine is excellent, with their flanking, going for cover when shot, and overall intelligence when spotting you.

With that said, this game is amazing, and going through the first time not completely familiar with the mechanics these gripes don't really apply. But on second playthrough, and even on hard, the game just felt easy. I hope somebody creates a mod to make enemies attack and move faster.

Jeff: "I hate you Jeff" sums it up pretty well. Most terrifying enemy of them all, but once you realize as long as you cover your mouth or have a respirator and know how to throw bottles, he'll never kill you. The only time he killed me was when I was trying to get the Near-Jeff Experience achievement (which I think is bugged) and I slapped his back after standing right next to him for over a minute. He immediately turned around and murdered me.

So I've played through the game twice now, once on normal mode, and the second time on hard mode carrying some dumb ass gnome. I'll never have that experience to play through the game again for the first time, and going through it the second time I started to notice some problems. Let me preface the gripes I have with a solid recommendation: it's really really good. The main gripe I have with the game is after you've mastered the gameplay, the game becomes easy. Really really easy. Let me give a breakdown of each main enemy and I'll explain. Barnacles: You just shoot them. If they are ever in a spot you can't shoot them there's usually an item nearby to force grab. Most of the time you aren't just teleporting/moving around willy nilly so you'll basically never accidentally run into them. Headcrabs: They take a bit of time to jump on you, and once they do you can simply move out of the way or block them with a grabbed object. Poison Headcrabs: While they are more tanky than their plain brethren, they still have the main issue where they take too long to attack and can be dodged. The poison effect they give goes away very quickly as well and from what I can tell don't drop your HP to 1 like in Half-Life 2. Armored Headcrabs: These guys are more interesting to deal with when they are on a zombie, as they will need more shots to take down, but once they are on the ground and you have either a. good aim b. a laser sight upgrade or c. decent reflexes, they die in one shot from the pistol, even on hard. Zombies: They move slowly enough that in most cases you can simply walk away. Reviver: This enemies mechanics are awesome, but once it's in a zombie the projectile the zombie creates is so slow it's almost pitiful, and the entire encounter boils down to "don't go near it, wait for the electric pulses." Antlions: Super intimidating but will rarely ever hit you since they move slowly, won't jump at you like in Half-Life 2, and attack really slowly. Projectile Antlion: This is a cool enemy, but every time you fight them there's a convenient place to hide from their attack. Manhack: Due to the excellent sound design and the fact that shotguns exist, these guys will probably never sneak up on you and die even on hard with one shotgun blast. Combine: These guys aren't pushovers, which in my opinion makes them the best enemy types in the game. The heavies with their giant footsteps, shield, and overall tankiness are a blast to deal with. The minigun combine are a bit bland, as once you hear them rev (thanks sound design, we don't deserve you), it's simply a matter of waiting for them to stop, then peeking out and shooting them. The AI overall for the combine is excellent, with their flanking, going for cover when shot, and overall intelligence when spotting you. With that said, this game is amazing, and going through the first time not completely familiar with the mechanics these gripes don't really apply. But on second playthrough, and even on hard, the game just felt easy. I hope somebody creates a mod to make enemies attack and move faster. Jeff: "I hate you Jeff" sums it up pretty well. Most terrifying enemy of them all, but once you realize as long as you cover your mouth or have a respirator and know how to throw bottles, he'll never kill you. The only time he killed me was when I was trying to get the Near-Jeff Experience achievement (which I think is bugged) and I slapped his back after standing right next to him for over a minute. He immediately turned around and murdered me.