Half Life is one of the most heavily modded franchises in gaming history, topped only by DOOM in the FPS genre, a franchise that inspired Half Life. Valve's technology has been used for an endless variety of games. RTS, FPS, Puzzle, Adventure, Horror, etc. But you know what you just want sometimes?
More Half Life 2.
And that's what Downfall is.
There's no super special mechanic, vehicle, visuals, or character. It is merely Gordon Freeman on a journey in the vein of Half Life 2: Episode 2, with a beat up vehicle, going around shooting Ant-Lion's, Zombies, and Combine. It's excellently paced, presented, and polished.
Had someone tell me this was cut-content by Valve, I would've said it shouldn't have been cut. There are a few sections which repeat Half Life 2 a tad too much, and due to not being changing anything up, it's unlikely to surprise anybody at any point.
But for Half Life fans, it'll leave you wanting more.