for example, if I have the first turn of the game, and my hand has, after drawing (hypothetically):
Trash a Keeper
Let’s Simplify
Mix it all Up
Empty The Trash

It seems like none of these can be “played”, so do I just discard a card, and the turn moves to the next player?

A: One of our fans pointed out to us that a wonderful quality about Fluxx is that there are never cards that you “cannot play”. They thought this was great because even the youngest player could pick a card from their hand, and never be told “oh, you can’t play that.” Of course, that card might not actually DO anything when they play it, but it’s still a perfectly valid play, and that person has not made an error by attempting to play an unplayable card. It’s very inclusive.

So there’s that: all cards are playable, even if they don’t do anything on your turn. Secondly, you are never allowed to “pass” on your play requirement, or discard cards, unless you are reducing your hand size as required by a Hand Limit (and this only happens when it’s not your turn, or at the end of your turn, after you’ve executed all of your plays.)

It’s true that what ends up happening when you play an Action card that has no effect LOOKS A LOT like discarding it, since you “do what it says, then put it in the discard pile”. So you do what it says, nothing happens, and then it goes in on the discard pile. But it’s an important distinction, because of the rule against discarding or “passing” instead of playing: sometimes you may be forced to play a card even though it might make someone else win.