I'd seen a comment somewhere that the devs learned all the wrong lessons.

I agree. The core combat of the game is still great, but everything else kind of sucks. The limited ammo I get, but it is too limited. The upgrades make perfect sense for the guns, but the armor and ruin points are kind of dumb. Ruins make some sense, but the armor... it is poorly explained, and I can't even find how to get there in the UI. And the upgrades seem kind of, well, dumb.

The biggest issue with the original is that replaying levels didn't really work - some levels were 3-4 hours long, but you couldn't save mid-level in replays. They fixed that well, letting you do all kinds of cheat codes when you replay.
But everything else new is kind of terrible.

But, take a fantastic game and add a bunch of terrible new stuff, while keeping the core the same, and you still have a good, fun game.