Command has, in my experience, been abused by DMs a lot over the years. Like an NPC telling my PC to "jump" when we were on ship. So I jumped in place and the DM just said "you know what I mean" and then made my PC jump off the side of the ship. Because apparently jumping in place or to the middle of the ship wasn't jumping. Since I was in armor, he said I sank straight to the bottom like a rock. Fortunately we were in harbor.I'm with the crowd that the 5e version of this spell forces the target to perform an action it can actually follow (so sleep wouldn't cause it to sleep but would cause it to fall prone and try to sleep).
Further - the target interprets the command - so (even though it's on the list of examples) something like "DROP" is problematic. The word drop can easily mean "fall down" not "drop what you are holding." So, for example, commanding someone with a crossbow to "drop" could very well cause them to drop prone and then shoot.
As for causing to drop concentration - what one word would even do that? Can't think of one off hand (if I heard "deconcentrate" I'd also likely grab my canteen and drink!) But if I could, it would likely just cause a DC 10 concentration check.
This was a public game and for whatever reason this particular DM was a dick to me from the very first game we played before I even had a single turn. But the moral of the story is there's a reason I limit the commands to things similar to those in the list because things can be broadly interpreted by the target to mean many different things.
If command can be anything just command the target to die.
