What should I check to ensure my level is enjoyable/not flawed before uploading? [closed]

I'll take a stab at answering this from the perspective of someone who often plays endless mode.

Give extra lives

When playing for high score in endless mode (or just trying to unlock costume pieces), extra lives in levels can make or break a run. They can be a fun secret item to hide in corners of your level to reward the adventurous player, or just freely given to players at the end, as long as you give players a chance to come out of the level a net gain of lives. There's no downside in being generous here, and if I have a chance to gain three extra lives from your level, I'll be a bit more likely to risk a few deaths on it as well.

Always show the next move

If I come up to a cliff where it's unclear if I'm supposed to drop down to a safe platform below, run and jump as far as possible, or wonder if I missed a door or warp pipe, I'm most likely skipping the level. Players should always have a sense of where they need to go, so if there's any section of the level where it may not be clear what you're jumping too, use coins to hint at where they should jump or place arrows to point players in the right direction.

Avoid surprise deaths

If I start a level or go through a door/pipe and die a split second later because I was expected to run/jump/dodge an immediate threat, I'll boo and skip the level on the spot. Give players a chance to see dangers ahead, and plan for how to tackle them. When I have 5 lives left, I don't want to lose 1 of them learning that I need to hold right + jump when I open the first door.

Give enough time to plan ahead

Speed run levels are popular and often a lot of fun, but when playing endless mode, starting a level with 20 seconds on the clock usually warrants an immediate skip.

Don't include sections you have to practice

If you make a cool new jumping puzzle that takes you 20 tries to perfect, then players at your skill level are either burning 20 lives trying to get past that one part, or just skipping the level.

Please keep in mind this is all only in regards to endless mode players. When I'm just playing hot or new levels, I love taking time to learn a particularly challenging level, and most of the above guidelines no longer apply (still no surprise deaths though!).