The Lobby Ambush in "Questionable Ethics". A quick but vicious fight against the toughest enemies of the game, which the player is given suitable kudos for surviving by the NPCs.
Scientist:[seeing the carnage] Look what you did!
Security Guard: Holy shit.
Scientist: I...can't believe what you did!
Keep an old, balding guard alive during "Forget About Freeman": After the other old balding guard realizes you're not marines, we get this exchange
OOBG: How many lives do you have, Jackson?
Jackson: None, if it weren't for my new buddy here.
Magnum ammmo is a little more plentiful in this game. As such, you'll be using it more often...and every time you kill a hostile Headcrab Zombie or military trooper with it—hell, just firing that hand-cannon, thanks to the updated graphics and sound—will make the player feel powerful.
Related to that, the satisfaction of plugging one of those HECU Assassins with it and getting the Dead Reckoning achievement is just amazing
Headcrab zombies. Yes, you read that right. If you don't kill them with a headshot, the headcrab will leap off the corpse it was controlling, making you waste extra rounds to kill it for good.
The Gargantuas are much more powerful this time around—you won't be able to just kill these beasts with a massive stockpile of ammo. You have to use the environmental hazards present to kill them when they show up, and pray you can outrun them long enough to get to said hazards.
The Tentacles you fight in the Blast Pit are a lot more sensitive to noise and movement this time around.
The Vortigaunts get a subtle one in the form of their electric bolts—they're much more accurate and quick to charge; if you don't keep moving or put them down fast, you'll get a nasty jolt.
The Houndeyes' shockwaves do more damage, and they blur your screen a little while a ringing noise fills your ears for a moment.
The Ichthyosaurs—they tend to lurk in murky water, so you won't see them until they're practically on top of you. Here's hoping you've got a steady trigger finger and nerves of steel...
On the human side of things, the helicopter gunships used by the military pack a hard punch, and can take punishment just as well as they dish it out. Bring your heaviest firepower, and find cover.
The human grunts get this, too. The commandos have all their original strategic maneuvers, plus they can run and shoot, now. Oh, and they're more accurate with their guns. They even have medics this time around!
Even the allies get some. While they still aren't as strong as the opposing forces, they're smarter.
Security guards will sneak up on enemies and put up a definite fight, lasting much longer most of the time than they normally did.
Scientists will use caution and tend to stay out of your way more often so they don't inadvertently die as easily, and they still heal, much better actually.
Many awesome moments from the original game return, but the CROWNING moment is a meta example: Black Mesa is a fanmade, updated re-release of the original Half Life using the Source engine. Despite using the same engine as the newer Valve games, it also has many features that those games don't, and is superior in many respects to most of what Valve has made. It was done by a group of fans on their own time. And it's free.
Also one for Valve: most video game companies would freak out if a bunch of fans started making a free, improved version of a product that they were still selling for ten dollars. Valve, on the other hand, put it up on Steam for download and said that they were just as excited for the release as everyone else was. A lot of people are taking odds as to whether or not the BM development team gets hired up by Valve.