Great first demo... this is some good stuff.
In this comment, I'll leave out what a lot of others are saying about what should "be in the game" like online multiplayer, tutorial, builds for other OSs, bots, etc because I know you've already been considering many of those things. Instead, I'll try to give some thoughts on what you can do to improve what you have here already, mainly the controls.
(Before I get into a ton of detail, it's worth noting that I'm definitely a mouse and keyboard guy, so those with more controller experience might not feel the way I did 100%)
I played for a bit of time, maybe 15 to 20 minutes. In that time I managed to get relatively comfortable with landing big punches on the dummy. Even after that time though, the rate at which I landed a hit felt inconsistent. I often wasn't able to determine what I did differently between a max-power hit and a scrape, or between a hook and an upper-cut.
Along those lines, it was a bit difficult to find the distance needed to land hits. I felt I was often either too far away or too close, and it was difficult to tell until I swung and missed.
I also noticed that I naturally felt far more comfortable swinging punches with the right side rather than the left. This might be because I'm personally right-handed, but it might also be because the lean-stick is on the right. I might be an edge case here, and it might not even be an issue.
I'm a tad concerned about how the mechanics of the game will translate to 1v1s, at least for new players. Like I said, in my first 20 minutes I barely managed to get used to punching a dummy, let alone balancing positioning, blocking, stamina, and punching in an actual match. Learning curves are acceptable and are welcome for many players, but many others might get frustrated if they are learning that balancing act in live games. I'd say that theoretically, the best way to address this would be to either simplify the mechanics or provide a way to practice them offline. Not just practice for punching, but for that balancing act the combat features.
The review sounds worrisome, but for real, it's a really good start. When you do land those hits it feels really good, and I could see a community building up around its combat mechanics. Overall, really great! Can't wait to play the next updates!