These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Special mention goes to the song that plays during the lobby fight in Questionable Ethics. Really makes the moment feel a lot more badass.
Another special mention goes to Black Mesa Theme, which beautifully conveys the tragedy of what happens at Black Mesa. Words alone cannot do it justice.
Additionally, allthreeparts of Inbound help to set up an atmosphere of foreboding for tragedy to come.
Demonic Spiders: The marines are a nightmare to fight. They're very tough, headshots are difficult to pull off, they happen to be very accurate, carry automatic weapons, and there are medics in every squad. Commanders and Shotgun Soldiers are even worse, with the former having more health and doing more damage, and the latter carrying fully automatic shotguns that can reduce a full health and armor bar to nothing in seconds. Also, have fun trying to follow usual video game logic and setting up a choke point where you can cut them down as soon as they step into your sights, only to lose the ensuing waiting game. Soldiers won't blindly rush forward into an area where they've lost men, preferring instead to set up defensible positions and wait for you to pop out. Similarly, they'll take cover to reload, and advance if you stop to do the same.
The Houndeyes went from being somewhat weak in the original to being quite dangerous in Black Mesa, to the frustration of some players. They attack very quickly, are difficult to escape from, come at you in packs and mess with your screen/audio.
Disappointing Last Level: The development team are well aware of the reputation the Xen levels had in the original game, and has vowed to solve the problems that plagued them. Whether or not they succeed at this goal remains to be seen, as the final chapters of the game has yet to be released. As it is at the time of writing, the game simply ends once the player goes through the portal to Xen.
Game Breaker: The Hivehand was significantly buffed to do a lot more damage compared to the original game. Almost to the point that there's no reason to use any other weapon after obtaining it due to its infinite ammo, and taking down enemies within a second or two, and that doesn't even include the fact that the Hivehand homes in on its targets.
Good Bad Bugs: Even despite the large amount of effort put into the mod, some glitches - either via oversights or from the Source engine itself - exist. Thankfully, few truly hamper the game in any way and some tend to do unintended things.
When fighting the helicopter in Surface Tension, if you run back into the room where you get the rocket launcher and then run back out, the helicopter is rotated ninety degrees to one side. It can't turn from this position, so it can't hit you, turning That One Boss into a walk in the park.
When a security guard gets within melee range of an enemy, they stop firing. This can be rather frustrating at the beginning of the game, when you're unarmed and reliant on a security guard to kill enemies for you. Thankfully, you can get past them on your own without taking damage.
Game-Breaking Bug: Partially, as the devs have stated that some players randomly crashing at various points in the game, whether always at a specific position or just from playing for a while, is not their fault, but the Source engine's from being a bit overstressed by the game.
Narm: While the voice acting generally avoids this, the marines' dialogue can be ... somewhat forced to say the least.
Crouch-jumping, which is required pretty much every time jumping is necessary, and requires two button presses rather than onenote and heaven help you if you need to perform a running crouch jump. Some wonder why the normal jump even exists at all when it's so completely useless.
The radiation static also gets a lot of hatred, mostly because it can obscure the player's vision a lot even in minor areas, can be triggered even when the player isn't being irradiated but is standing near a radiated area, has been reported to cause headaches and nausea, and, most importantly, unlike the tinnitus and chromatic abberation effects it cannot be turned off in the options.
It's portrayed as being much more realistic here, but vent crawling in general has become greatly despised in Black Mesa particularly because it's so slow to move through the vents compared to the original game where you were practically running at top speed through them. Most who have played Black Mesa believe this to be the reason the vent maze in "We've Got Hostiles!" didn't make the final cut.
That One Boss: The helicopter in Surface Tension, right after you get the rocket launcher. It does a lot of damage, it maneuvers around your cover pretty well, it can unleash a hail of rockets to kill you with splash damage, and it takes a lot of rockets to bring down.
That One Level: The marine ambush at the end of "Questionable Ethics". Two full waves of soldiers who can flank you from at least 3 different directions, plus two more soldiers watching through a roof window to make sure you have a hard time maneuvering to better cover. For bonus points, this encounter wasn't in the original Half-Life.
Another good candidate is the battle near the end of Surface Tension. A VTOL constantly drops waves of soldiers on you, and there's a group of Alien Grunts who can also do pretty good damage. In the original Half-Life, you just had to reach a turret and kill the VTOL and everyone else with it, but in this mod, you have to physically pick up the missiles and carry them to the turret. This is slow and difficult to do while under fire. Though the section is much easier if you saved rocket launcher ammo or use the ammo around the area, as three good hits on Normal takes the VTOL down.
Many of these issues are over small things, such as weapon placement, nerfs/buffs to weapons and voice acting.
The largest debates however, are over the "On a Rail" and "Surface Tension" chapters, and the vast cuts made to them.
The former was subject to practically an entire restructuring of it's content with only a few areas from the original game being remade like the starting rail area that's been flooded, the areas around the security guard office and the final outdoor areas around the rocket.
With the latter, it stayed relatively the same throughout...except that about a 4th of the level had been cut out. Specifically, the areas that take place between being shot out of the Snark vent and the Gargantuan chase through the garage tunnels.
It's not as much as either the On a Rail or Surface Tension debates, but on rare occasions, you'll see a complaint crop up regarding the vent maze being cut out of We've got Hostiles!
Completely invulnerable Gargs get some flak, too. It was a fun challenge to kill them with explosives.
This can be changed with the console command sk_gargantua_debug_allow_ death 1. However, under this command, the Garg will die in one hit from any explosive (bullets, Tau Cannon shots, Thornets, et cetera still do nothing), so there really is no way to get the challenge of the first game in the Black Mesa Gargs.
There was a controversy over the M4A1 and M9 being inexplicably replaced, with many people complaining. The only official response was a developer telling them to stop complaining about the change.
The Glock and MP5 were the weapons that were in the original Half-Life by default. The M4A1 and M9 were added by the HD pack later.