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.

{insert bad joke here}

But in all seriousness: don't over scope and spend time on polishing. Get people to play test early and often. Also, make sure your itch page looks good (with a good thumbnail as well). Basically, do the opposite of me :P

Something else you should be doing: use "karma" sort to make sure you are rating games where the creator is rating more games than he/she is getting and to leave feedback.

Doing this means that you are rating people who are rating themselves, while also being in need of more rates and leaving feedback also puts your game right in their view (as itch automatically puts a link to your game in your comment). This means that when you leave feedback you are automatically advertising your game to people who rate (because the sort on karma) and as you left feedback there is a higher chance that the guy you left feedback for rates your game in return.

for me it worked finishing the game from start to finish before the third day (having it in a playable state up to the screen that tells you, you win). The third day was dedicated only to polishing, which most of the time ends up being more than what I expected, adding sound effects, testing, adjusting the difficulty and level design, correcting bugs, adding a menu, player interface etc. Also making the itch page.

My biggest take away from this jam is that if a level or mechanic causes me (the Developer) have to try or difficult for me to beat, its probably best to tone it back as most people haven't, nor will have, the experience and know how I do developing the game.

Aka: make levels easier than normal for you as the Developer of your game.

If you're making a platformer, always, always, always make sure it feels good to move your character around. 

There's a somewhat popular urban legend that at the start of Super Mario 64's development, Shigeru Miyamoto wanted to make sure that Mario felt fun to control just by himself.  To accomplish this, he simply tested Mario's movement in an empty white room and polished it until it was perfect.  And it shows!

If you're not sure about your physics and movement, make sure you get feedback.  Even if you are sure, make sure you get feedback.  Something that may have felt normal to you may feel weird to somebody else.  

And remember: don't take all criticism equally.  Oftentimes criticism is helpful, but keep in mind that not everyone shares your exact vision for your game.   After all, if Miyamoto gave Mario a rocket launcher in Super Mario 64, the experience wouldn't feel as unified as the final product*.  Learn to discern between these types of criticism. 

*On the other hand, this sounds amazing.  Take notes, Nintendo.

• Really spend your time polishing your game. Building levels can be done another time. Though if there are only a few hours left and your game has absolutely no levels, then I suggest you get started. Unlike me, who foolishly threw all of the game's levels together in the last hour of my time budget. That was not fun. 

One good game design trick I've heard of is to strip the game of its level design, visuals, and sounds, (essentially turn the player into a square and trap them in a box) and if the player feels good and fun to just move around and can keep your attention even with no real objective or challenge, you've got yourself the foundation of a great game.

• You have to make them interested in your game. What good is a great game if the player doesn't notice it among all the others? There could be some true gems hidden here and there that don't get noticed because of the lack of a cover image and a catchy title. That's why marketing and communication are important for your game to get seen and played.