I’ve played a lot of games that use Z as (one of) their action keys. For someone using a standard US keyboard layout, this might be surprising to you, but this key is not necessarily in the bottom left corner on your player’s keyboard. As long as you only use one action key, that is okay, because even if the player has to put his finger onto the center of the top row of his keyboard, most should be able to manage that.
But, consider you have two action keys, say “Jump” and “Fire” and you chose to use Z and X for that. Now this is very uncomfortable for everyone with a French, German or alternative Italian keyboard layout, where the Z key is in the top row of keys (and for the German layout it is even moved to the center of the top row, which is almost impossible to use).
Of course, I can just change my keyboard layout to US and that’s it, but not every player has the knowledge or the will to do that and you don’t want to limit the number of people who would be able to easily play your game, right? After all, there are about 2800 other games to chose from, you want to make it easy for your players.
So, which keys should you use? Take a look at this chart:

This is the ISO recommendation for Latin keyboard layouts. The black keys should be fixed on all standard layouts (i.e. not on Dvorak, Neo or de-ergo or Bépo layouts, but I think those are rare enough and their users should be used to changing their layouts regularly). What we can also see is that WASD as direction keys is not necessarily a good idea. It works on US and DE layouts, but everyone with a French layout gets in trouble with that.
So what should you use? In practice, XCVB for action keys has been used a lot and seems to work on many keyboards without Ghosting (another thing to keep in mind) and there are lots of gamers out there who prefer ESDF movement keys, so it should work quite well in practice.
Another thing: Some layouts (like French) can use digit keys only with shift, so better don’t use numerals for important actions.
The best solution would of course be to have configurable keys, but especially in a 48h contest, you will hardly have enough time to add that. I did it in my LD31 entry though and got a bit of praise for that, so maybe it’s a good idea to do if you want to squeeze the last bit out of your Overall score. Whatever you do, consider making it easy for non QWERTY keyboard users. Thanks for your attention 😉