So I shelled out the bux for the Alpha and I'll give my impressions here:
This is the most playable Alpha I've ever seen, not because there's an actual
game per se but because the engine is so modular, easy to work with, and well designed. The Overgrowth Engine is the kind of stuff that could give Source a run for its money in terms of modability (assuming that it ever becomes popular).
As far as what can be done in the Alpha, there are a few challenges set up (no objective recognition) and a Gladiator's Arena where the opponents are procedurally generated based on your skill level, with both armed and unarmed fights. Other than that the real core of the gameplay is fiddling with the level editing tools and fooling around with the core combat & parkour gameplay.
Doing more requires getting involved with the modding community, and even being able to setup fullscreen or change the settings necessitates a 3rd Party program called SUMLauncher. Getting SUMLauncher involves a convoluted process where you have to create a Humble Bundle account (purchases not necessary), then use a Steam cd key with your Humble Bundle info to authenticate on the Overgrowth website and use the resulting Overgrowth verification key to authenticate with the SUMlauncher. Even being able to see the modding or Overgrowth subforums on the Woflire forums is impossible without going through this authentication process.
SUMLauncher is a neat utility tool that gives access to a lot of impressive and not-so impressive community content. Right now it's mostly just maps, characters, weapons and other objects. Anything like a full conversion is going to be a distant future thing. Now if only the program would let me lower volume settings so I can listen to podcasts while bunny punching.
It's actually very diffiicult to pull off a foot-sweep properly because the AI does a really good job of blocking or dodging the sweeps. Because the fighting system is contextual, it can be fairly difficult to pull off the exact kind of attack that you want, and crouching down to pull off a footsweep is a huge red flag for the AI to respond to.
Damage modeling is also a skeleton-based system as opposed to any kind of HP modeling. You can take a lot of unarmed blows and a few slashes from bladed weapons (if you're lucky), but anything that would break your bones, snap the neck, or stabbing attacks are fatal. What will really get you or an opponent is getting smashed into a hard surface. It's possible to break your neck just from flipping at a low altitude. Getting hit while you're not in a stance or blocking will ragdoll you. A roundhouse kick to the face is Goodnight Gracy.
The engine also effectively models mass and other parameters for in-game characters, so the potential for a good action RPG is already there. Rabbits are the weakest but also the most agile, dogs are the toughest and wear the most armor, but wolves are gigantic and their attacks can't be blocked.
The AI also adapts to your moveset, so the longer it stays alive the more it anticipates what you're going to do and needs to be thrown off. Bottom line is that combat feels fresh, and could be the basis for a lot of really sweet total conversions or standalone games once the dev tools are released.