Singleplayer
I've heard it said that there is no technical difference between singleplayer mode and multiplayer mode, but I find this not to be so, for a number of reasons. My runfast mod doesn't work properly in singleplayer, for example. A modding error, perhaps, but a good illustration nonetheless. <-- Definitely a modding error.
The biggest issue I take with this is that the player's name is "singleplayer". Just because it's a single player mode, doesn't mean that single player doesn't have a name. "Owned by singleplayer" <-- That one hurts.
Unbindable keys
I have been playing first person games since 1993, and for as long as I've been using the mouse (1994), and for as long as jumping has been available (1995), I have used the second mouse button for jumping. I know space is a pretty obvious default, and that many people use and prefer it. But I don't.
I think I just needed to vent. Thanks for minetest!
PS. I realize these two points have been brought up before, and that this post may be in duplicate; but feel free to post your own pet peeves anyway, please.