The core of fun for me is the Learning. This is why every time I add something to my level I ask myself "What am I teaching here?" and "What am I asking the player?".
Another important thing is the Pacing. I always try to have some safer areas or strong powerups after difficult parts, so the player can catch their breath and try new stuff. This is that dip in the curve, and that's specially useful to make the next part feel even harder without actually making it hard and frustrating the player.
Thanks a lot to Matt Thorson for helping me with this one!
Also, thanks a lot to all my Patreons! You are making this tutorial series possible :)
PS. About the streams:My plan is to start streaming later this week. I just got the last month's payment and invested it in an extra monitor so I can stream properly, I'm waiting for it to arrive!