Does Black Mesa feature new weapons?
Are there any new features for weapons?
With exception of the Magnum (which has now an optional iron-sight like its version in Half-Life: Deathmatch ) and the Crossbow (which features a scope instead of a simple zoom), the weapons were only “sourcified” and rebalanced. No new functions like free-aiming, new secondary attacks etc were added.
Was the Gravity Gun included in the mod?
No. The Gravity Gun doesn't fit into the Half-Life storyline and therefore wasn't included. Also, you will not be able to get it through cheat codes because the coding for it has been removed. 
Can I switch between the MP5, MP5 with the red-dot sight and M4 models via console command?
How grenades work in Black Mesa? Like Half-Life or Half-Life 2?
Grenades mix both styles; they behave like the original Half-Life in terms of timing and cooking but like Half-Life 2 in terms of throwing and rolling. 
The model of <put the name of a gun here> isn't exactly as the real life/Half-Life one! Change it now!!!
Quoting a Black Mesa developer himself: "BM is not reality. We are not gun nuts and, despite it being a first person shooter, Half-Life was never about the guns. Not the realistic ones, anyway. " So, if a gun (or marine/guard uniform, or science prop, or whatever) isn't exactly like in the original Half-life, or lack some detail that exists in the real life version... get over it. These changes are time consuming and demand a lot of work .
NPCs & Enemies
Are my favorite characters from the sequels seen in Black Mesa?
Some of them are, others aren't. Barney, Eli Vance and Isaac Kleiner are seen. You will not see: Adrian Shephard, Gina Cross, Colette Green, Alyx Vance, Dr. Breen, Dr. Mossman, Dr. Magnusson, Dog, or any other character who are only made known to the player in HL2 and Episodes.
Magnusson isn't in the mod, but how about the debacle with his microwave casserole?
The reference to the dialogue with Dr. Magnusson in Episode 2 is still in.
Can I kill/hurt friendly NPCs? What happens if I do it?
Yes. For more details, see NPC Alignment System.
How many security guard / scientist / HECU marine models have you made? Are there a lot of twins?
Black Mesa has NPCs of diverse age, ethnicity and gender (except for guards and marines, which have no female versions). The textures/models for generic scientists, marines or security guards never change (except between pre and post disaster), but the head/face combinations and body adjustments are completely random and will be different on each playthrough.
Are there HECU Marines with ski goggles and balaclavas?
Do the Vortigaunts gain attacks from Episode 2?
Can I find new enemies/Half-Life cut enemies in Black Mesa?
Did you cut leeches from the first release?
Yes. They were originally meant to attack the player like in the original Half-Life, but play-testing revealed that this was highly confusing to the testers, especially in dark water sections .
Levels & Map Structure (First Release)
Is the Hazard Course featured in Black Mesa?
No. The little HUD hints (like in Half-Life 2 and the episodes) you receive throughout the chapters Anomalous Materials and Unforeseen Consequences made this separate training section unnecessary. 
Where do I start the game?
You will start the game in the Black Mesa Inbound Tram as in the original Half-Life.
Has Black Mesa similar map structure and load times compared to Half-life?
All of the levels in Black Mesa are much larger due to the Source Engine's improved capabilities. In general, Black Mesa has more cut and reworked sections than new areas added  -- and many chapters that used to be several maps are condensed into less individual maps  -- so there are less load times. This is designed to interrupt gameplay less and make for a more contiguous user experience.
How long can I expect a playthrough of Black Mesa to take, on average, compared to Half-Life?
It is almost entirely up to you and how you play the game. If you take your time and immerse yourself, it will take longer than if you try and speed-run it. You can expect at least 8-10 hours of normal gameplay from the chapter Inbound to the end of Lambda Core , or less than 3 hours on a speedrun .
Xen (Second Release)
The game ends in Lambda Core!? What happened to Xen??
The Xen-related chapters were postponed to the second release. See more info about that.
Have you made any significant changes to Xen? What changes?
Are Achievements possible in this mod?
Which difficulty levels has Black Mesa?
WTF... I can't jump high enough in this game!?!? Is my Black Mesa bugged???
Nope. You simply aren't using the crouch-jump (CTRL + SPACE). The crouch-jump is an important gameplay aspect to solve the jumping puzzles, and without it you cannot progress too far in the mod.
Can I find the Half-Life's <insert the bug/exploit name here> bug in Black Mesa?
As the Black Mesa is Half-Life rebuit from the ground up on the Source Engine, it is unlikely that the same bugs found in the GoldSrc Engine are present in the mod.
Have you used the music (or remixed pieces of them) from the Half-Life soundtrack?
No. Black Mesa features its own soundtrack.
So is the music scripted in a way that it ends when the fight ends?
The soundtrack is scripted and placed appropriately, but does not change dynamically, like it does in L4D with the usage of the Director . The music isn't triggered by combat or anything that complex .
Can I hear more music/get it in mp3 format?
You'll find all of the released (and unreleased) tracks in the Music & Sound section.
But I prefer the HL original soundtrack! No way to change it?
It is possible to replace the new music with the old ones, but you'll have to do it yourself. 
- Easter Eggs: They are mostly related to Black Mesa's development. Some of them are based on the original easter eggs from Half-Life, but aren't exactly the same.
- HEV Suit Helmet: No helmet for Gordon Freeman.
- Gordon model in 1st person: As in all Half-Life series, you can't see his legs in first person view. Also his body don't cast shadows on the world .
- Gordon model in 3rd person: There is no model for Gordon in 3rd person. Like in HL2, there is only a "grayman" model instead .
- Mirrors: Mirrors (the in-game reflective surfaces in which the player can see himself, not the download locations) are not featured in Black Mesa.
- Soda Cans & Soda/Vending Machines: The soda cans heal the player like in Half-Life, however the soda/vending machines cannot be destroyed (only inactivated).
- Continuity with Gearbox Expansions: The expansions for Half-Life (Opposing Force, Blue Shift and Decay) are largely ignored. The Developers didn't make Black Mesa tie in more with these games. The only known exception to this rule is the location of Barney Calhoun during Inbound.
- Content from HL2 and Episodes used in Black Mesa: The majority of the content used in BM is made by the team. But BM also has few contents imported from the Orange Box SDK Shared Resources, reskinned and/or adapted to fit the mod standards. Some of them are: