Sadly the alien and experimental weapons that Gordon used in the original Half-Life are, for obvious reasons, gone. Possibly the best thought-up weapon in the game is the Barnacle Gun. You can either pull baddies toward you or use it as a grappling hook. The single players maps are awesome. You can see that Gearbox put in a lot of effort into each map to make it perfect.
Every single area has a purpose in the general flow of the game.
The annoying scientists from the original training mission have been replaced by Drill Sergeants that scream at you throughout the whole training mission. The training mission is also delivered in a very humorous manner with such great activities as being shot at by a firing range. The new aliens are much bigger and stronger than the aliens in the original, add in a few new bosses and you have yourself quite the party. Also the Black Ops from the original are back, and this time they brought their boyfriends, who are thankfully slower but pack a mean punch. The AI of your Grunt friends is less than spectacular, but I guess it will have to do.