Good thread. A few thoughts:
When choosing aesthetics it is better to be subtle than flashy. Scrolling graphics, heavy particle usage and clashing colors will cause annoyance and eye strains.
I agree with the general principle that graphics should never detract from gameplay, however what you say here isn't really true. It's very possible to pull off flashy graphics, contrasting colours, heavy particle usage & scrolling graphics without them causing annoyance or eye strain (though probably not all at once). The underlying principle here is solid, but I think this is less of a rule and more of a preference on your part.
Export your game as a standalone exe or a zip, and not an installer.
I think when people do this it's just because they haven't worked out that GM:S picks NSIS installer as its default export type; it's worth mentioning to newcomers that you can click the drop down to select a runtime application (.exe).
Don't include inappropriate content that puts streamers at risk for being banned. It may be funny or artistic, but being able to stream games is more important than that.
Though, to some people it's not. This is a good warning to those unaware, but for some (including myself) streaming ability is far less important than artistic coherence.
Playtesting is super important. If you spend 100 hours making a game, you don't want a few segments to be huge roadblocks or a mechanic to be confusing. Spending like 5 hours working with playtesters and changing things can make your game be twice as enjoyable, not exaggerating. It can be easy to overlook playtesting when you're finishing your game and you're exhausted, but you absolutely have to have other people play your game and point out things you can't see. Hm, it might be nice if someone wrote a longer piece about playtesting fangames.
Great point. To expand on this though, be discerning about who you pick to playtest. You want a good slice of skill level from your testers to ensure the game is interesting enough for hardcore players but not impossible for novice players & that the difficulty settings reflect this accurately. Obviously this isn't true if you're purposefully making a very hard game, but I think most people want to open their games up to as wider audience as possible.
Another thing to be mindful of when picking testers is if you're exclusively using friends. Sometimes it's hard to be fully honest with someone when you like them and don't want to hurt their feelings. If you think your playtesters might be holding back, push them to be more critical and try choosing people you are less close to in the future. Sometimes streamers like Stonk & Paragus can be good for this as you're getting their live reaction (though the chat should be taken with a pinch of salt in situations like this).
Also a longer piece on playtester both for what to do if you're a playtester & if you're the creator would be cool, yeah.