Im not a fan of Mold's Challenge either. They are 90% jumping challenges.
while i agree that it should probably be labeled 'jumping challenge', it isn't. it's still a good challenge, and has interesting tricks.
tbh a good challenge just needs more variety than most new challenges these days: too many times i'll join a challenge server to see "oh, choose a door in this level 8 times! you might find the right door!". there is 0 skill involved in that.
i'm not a big fan of click challenges but meh, a well made one can pull in a lot of players.
a good ping is always a necessity.. joined a challenge today that had two levels based on timing, which is pretty hard to do when your ping is consistently above 1000. needless to say, i told him that he needs to either lower player count or get a better isp to host.
and finally, you need to draw the right audiences. naming your challenge simply 'challenge' can be super misleading, as i said at the top of this post: i could join a challenge, expecting it to be something involving, say, jumping, only to find that it's a click challenge! oh no, i then leave the game in disappointment. basically, name your challenges something relevant; you'll attract more people who are interested in that kind of level design (ie i was hosting a grapple rope challenge; people who came were not complaining about the lack of mechanical jumping skills required to beat the challenge) and instead you'll get better feedback on your actual levels.
