Black Mesa's design is where it really shines. The game looks absolutely beautiful, filled with detail and polish. Somehow, Black Mesa manages to both innovate, with new and varied level design, while still remaining close enough to the source material that one will be able to recognize most areas and appreciate the work put into updating them. The layout of the game's maps have been modernized, trading straight corridors and occasionally illogical room design (such as the famous box room, which seemed to serve no purpose), for a more natural, convincing and immersive research facility. All of the locations are of course also filled with detail, including desks, chairs, computers and whatever else might belong.
The Anomalous Materials lab's lobby, as seen in Black Mesa, and in the upper right corner, as seen in Half-Life
The characters which populate the game are equally well made. NPC scientists, security guards and soldiers look good, and no two look alike thanks to the game's Face Creation System. The friendly NPCs are more realistic than in the original game, speaking more frequently, often making humorous comments. Aliens are equally well designed, being familiar to their original look in Half-Life, but updated for the modern source engine. Especially the iconic headcrab-zombie has had a lot of detail put into it and looks truly frightening.
The updated level design looks gorgeous, and your goo-covered hands just add to the experience
The game's user interface looks exactly like in the original game; in the bottom right corner you have health and hazard suit stats, and in the lower right corner you have weapons and ammo information. The only major update is on the player's character model (or hand and weapon model, as it were), a fantastically entertaining design aspect: In order to make the experience more immersive, your player model interacts with the environment, and you will often find your hands and weapons covered in slime or blood as you kill your enemies; It's very well done and really cool.