Oh wow, this has soooo much potential ! I can feel the lack of experience, but you got the right mood and some interesting ideas. You asked for constructive critisism, so I'll try to do my best, but keep in mind that, despite its flaws, I enjoyed your little game =).
- Graphics are **really good**. For a 72h game jam, I won't ask for more frames in the animations or whatsoever, just the fact that you made your own graphics is **awesome**, and I really like the main character. However, the *visual feedback* isn't that great : enemies change color when hit, but for some enemies (like the giant spider at the beginning), the color didn't change after some hits, so I didn't even know if my hits were registered. I don't ask for a specific animation here (your artist already had enough work to do =p), just the enemy bouncing back (or up, since the character attacks upwards) when hit is enough ^^.
- **There is no sound**, and that's a huge problem. Okay, there is no music, but I can live with that. The most important point is the lack of **audio feedback**. Again, when I hit an enemy, it should make a sound. Take any sound from internet and use it, I don't really care what is it. Rickroll me, play a "Hey, Listen" sound, it's okay, but something xD. That, plus the weak visual feedback, makes the combat feel... Empty. There is a cool animation for the attack, but it don't feel when the attack connects :/.
- I feel like the rune system is really interesting ! However, without proper explanation on how does that work, it's really obscure. I just drag-and-dropped randomly and was happy to jump higher xD. **You can have the best features in the world, if the player doesn't know how to use it, it's useless**. I am aware that in-game tutorial is long to implement, but just some explanations in the game description would have been enough !
- It's a personal preference, but I really don't like to use C to dash. To press C, I either need to release the D button (which is bad because the whole game is about going to the right), or the spacebar (which is bad because it cancels my jump if I want a midair dash). If you can, it's always better to map the controls so the player uses **all its fingers** before mapping two actions on the same finger. In your game, my pinky is unused, so it would have been more convenient to use "shift" or "control" for the dash. Don't think "button mapping", but more "finger mapping" =).
- There is no **real ending** : you just kill the boss and... That's it, you're softlocked. Again, I'm not asking for a cutscene or whatsoever, just a text saying "Hey, you finished the game, GG !" is enough. Here, I just felt like something should have happened, but didn't. Even now, I'm not even sure I really finished the game x).
- There are some bugs and oddities here and there. Dashing while sliding down a wall is glitched, and bonking a ceiling doesn't have any effect (the character should drop instantly instead of staying in the air), to name a few.
- Why is this a development build ? Not a huge deal, it doesn't change the game itself, but it's weird ^^.
And... That's it ! Again, that's a huge wall of text focusing on your weak points, but **don't get discouraged** ! This is a **really good first finished game jam** ! With more experience, you'll be able to implement more things in 72h, i'm just pointing out what should be (in my opinion) your next priorities =).
Keep up the good work, and see you at Ludum Dare 49 to see how you improved !