Here is a dump of my most important.
The most basic? BUILD FOR EVERY PLATFORM!
Linux, Windows, Mac, and if you can, WebGL.
"Linux has like 2 us-" Not for game devs. Programmers are significantly more likely to use Linux than the average person, and some Unity games require to go through hell and back to play with WINE for Windows-exclusives on Linux and Mac.
Also, I'm not downloading your game 99% of the time, I have better things to do than wait for it to install and risk viruses that don't apply for me because you didn't build for Linux (god damn it!).
For some games, WebGL is not an option. That makes sense, in this case, make sure you are building for all platforms.
Make sure you're careful with WebGL. Due to an oversight, WebGL for our game runs terribly because I forgot to optimize it for it.
"But GMS2 costs lots of money to export to htm-" I don't recommend GMS for jams. This is a personal preference, but I don't wanna rewrite features available out of the box on every other engine for a 3 day challenge. You may feel different and in general, GMS is an excellent engine, I just hate it for jams. Godot, Unity, and UE4 (although - UE4 is a bloated mess and sucks for anything relating to 2D) allow for building to the web for free. GMS is overpriced.
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Good thumbnails -> More people click -> More plays.
My game this time around had a high CTR compared to my previous jam submission, because of the thumbnails. A lot goes into a good thumbnail and this is an oversimplification, but make sure they are simple and not just modified screenshots, and display the title well. Look at some steam top sellers for good ideas on what makes you click.
Anyway, let's look at some real-world examples.
This thumbnail scales well without looking ugly or blocked out. There is quality art behind it instead of just taking a screenshot. The character shows an interesting design and do I need to explain further?
But with all art forms there, if you want to succeed, you need to have some degree of originality.
... Speaking of originality, you guys made over fifty platformers where the main gimmick is you can jump on corpses. This concept was one of 5 on my death list.
.. what is a death list?
Basically, spend 5-10 minutes brainstorming ideas. Look at that paper, take all of them, highlight the most interesting, and then throw the paper away and make sure never to use any of those. Who cares if one of them has a yellow background. Then come up with an idea not listed on there. You will have more unique ideas than if you just went with your first.
Again on originality, let's go back to art.
Simple colored shape games are so generic and overused. Sure, they're pretty and quick to make, but you get tired of them.
Take a look at Hinoken's art, this exceptional submission had a good thumbnail if you want to go back to that, also it is unique although I can't confirm how they got their idea, and has great graphics that are refreshing after Boxes With Bloom Repetition 5345
This submission did incredibly and it's just a fun game and I absolutely love it.
But what if you can't draw?
I can't. I got into a Team™. Although many prefer it solo, working with teams allows you to make something better than 1 person can.
My artists drew and animated much better than I ever could! No friends? Jonas's server has some people more than willing :)
You can also use other people's assets. Not a bad thing - assets can look great if matched well.
Then there's the generic quality over quantity which yeah we're not playing your game for 30 minutes, we're playing it for five and that's if it has a good hook.