Is it accessible?

I think this is an important question to ask early on. Your first prototype will probably need a bit of extra information before the player can try it, that’s no problem. But as you move on you’ll want to make it more and more accessible, without needing to give the playtester extra information. If however, you keep having problems with this, it might be better to drop or drastically change the game or its mechanics. Your game needs to stand on its own in the end and if it can only do that with loads of added information or tutorials, it might be better to avoid the hassle. Unless of course, the game and its mechanics are so interesting it’s all worth it. But that’s something you’ll want to decide on early on.