Characters and Environment:
New features include the addition of female scientists, an excellent soundtrack, iron sights for certain guns, and more interactive elements. Source's physics engine is used to good effect -- jump from suspended crate to suspended crates, and the storage containers realistically rock to reflect your movement.
The new scientist dialog is hilarious, enemy turrets can be picked up and repositioned, and Schrödinger's cat even offers a cameo appearance.
Weaponry, Enemies and Combat:
The list of weapons mostly mirrors Half-Life, but there are subtle differences in capability and placement. The secondary fire mode of the Glock 17 is gone, and the crowbar isn't the first weapon you encounter. The first zombies and headcrabs must be taken out with flares, which weren't available in the original game. Zombies will fling things at you, as they did in Half-Life 2, and igniting them with flares will trip the emergency fire suppression system depending on where you are in Black Mesa.
The 357 Magnum shows up much earlier in BMS; it's the second gun you'll find (in Half-Life, the shotgun came first). All of the weapons retain their original characteristics -- the 357 packs a massive punch, the Glock is best for long-range fire, the MP5 is a great all-around option and the underslung M203 grenade launcher delivers an immensely satisfying explosion.
Just a Dash of Half-Life 2:
Technically, Black Mesa is a total conversion of Half-Life 2, but the authors chose to flavor Black Mesa with HL2's capabilities, not dump them in wholesale. There's no Gravity Gun, and no way to port one in. Characters that weren't in the original game don't make any appearances here, and while Source's enhanced physics are used well, the implementation is subtle. Certain monsters, like the Zombie Security Guard, are new additions to the original game, and some creatures have had their powers tweaked. Vortigaunts, for example, charge much more quickly than in the original game.
Subtly reworking a beloved classic is far more dangerous than a straight port. Black Mesa chose to clean up, modify, and amend Valve's classic design, while keeping the memorable scenes and essence of the original. From what I've seen thus far, they've succeeded brilliantly. The original tram ride, the doomed guard just outside the entrance to Sector C, the cry of "Don't shoot -- we're on the science team!" -- they're all here. Environmental puzzles and little side rooms with upgrades for the eagle-eyed? They're here, too. You'll find helpful guards and scientists with ammo and a bit of healing to spare when you need them, and it's up to you how much trouble you go to when it comes to saving their lives.
Sure, there are a few rough spots. The cube-map reflections don't always work perfectly, the military voice-overs are a bit flat, and you can't hurt zombies or headcrabs by throwing trash cans or crates at them. None of it matters. Black Mesa is what Half Life Source could've been. It tweaks and yes, improves on the original game, while keeping everything that made Half-Life great. If you liked HL in 1998, you're going to love Black Mesa. If you couldn't get into Half-Life due its age, you'll still like Black Mesa.
In fact, a recent survey has proven that the only people who don't like Black Mesa are card-carrying members of PETH (People for the Ethical Treatment of Headcrabs). The rest of you -- get going. There's a crowbar with your name on it and the G-Man has his eye on you.