Two basic rules of gameplay
Gameplay for a world is like melody is to music. It is the soul of the level and the people who come to your world are usually looking for gameplay. So gameplay is important!
How to make good gameplay? I have interesting news for you: There's is no real "good" or "bad" gameplay! Anything can be good if the one playing is enjoying it. I think when building worlds the most important thing is that you enjoy the world yourself.
Here's step #1 for good gameplay: Recognize what you like and dislike!
Go play some worlds you like and worlds you dislike, and try to find out what you like/dislike about them. I myself rarely create difficult worlds, because I don't like playing difficult worlds myself. In other words, it's impossible for me to create a hard world that I would recognize as a "good" world. With easy worlds however, I can feel if I'm enjoying the world or not.
Step #2 for good gameplay: Play your own gameplay, and check if you like it or not! ...if not, edit it!
Seriously, if you don't like your own world, what's the point? If testing your world feels painful, something is probably wrong. If that's the case, try figuring out why you're not enjoying your own world and edit it to be better!
Small gameplay ideas/motifs
Ever thought of creating gameplay based on simple motifs? Many minigames are made by repeating one idea. An example from Lictor's EE Legend:
This minigame uses the idea of bouncing from an arrow wall. You can see that you have to to bounce two times in a row to get enough speed for winning the minigame.
A motif can really be anything from a single jump to a complex... thing. Get creative! Also, the motifs don't have to be repeated of course. You use just one motif or put different motifs together in a minigame too.
I think if Lictor had made his own tutorial, he'd have talked something about motifs. I never even thought about them before I heard it from Lictor you see!
Consistency in gamplay
I once asked Theditor to teach me how to make minigames. The thing I best remember him say was that gameplay needs to be consistent. What this basically means is that you should avoid gameplay that requires pixel perfect movement. An example minigame with bad consistency:
You can build this to your own world and try it out, it feels horrible (To me at least). In order to win, you have to do such a perfect edge jump that it ends up being luck based. Luck based gameplay is what we're trying to avoid when making our gameplay consistent. So avoid too perfect jumps and such.
There might be people who enjoy luck based gameplay, but I'm quite sure most do not.
Minigames
When you hear the words gameplay and EE, the word "minigame" comes into your mind, right? Most EE worlds divide their gameplay to small sections, which we call minigames. I think this is actually quite weird! When I started playing EE and creating my own little worlds, I didn't make anything like minigames. I just built - gameplay, I guess? Then I found a friend who showed me how minigames and hook jumps work, and that's where I said goodbye to my childish creativity.
No but seriously, why do we always make minigames? We have the option to make continuous gameplay like in most platformer games, but instead we make these seperate challenges. Maybe it has something to do with the gameplay being so hard that it'd be too painful not to cut it into smaller sections. Who knows!
I'm just here to say that minigames is one way of building a world, but it's definitely not the only one. You don't need to build your gameplay in the form of minigames for it to be good!