Nothing interesting in the game that could become an objective.
No threats to the player other than the player himself.
Almost no audio.
Combat is just clicking things.
Created by cargo-culting all the design decisions of Minecraft, but not executing them as well, this game is bound to bore you to death. Mine ores, make blocks out of them. Make huge ghost towns where the only interesting thing to do is to find the tallest building, climb it, then jump from it to your death. You can also pick some flowers, farm some cotton and make crude sprites. Or you could write expletives on wooden signs. So much fun!
Travel vast landscapes while thinking of all the cool things you would do in them, if they were implemented.
Explore dark dungeons for treasures such as: used shovel, a few cotton strings, some old apples and a bar of iron. Don't worry, nobody is guarding that anyway.
If you're feeling really adventurous, you can duel another player to the death. Your fate will then be decided by how many apples you are carrying, and how fast you can devour them while rapidly clicking the other guy.
You could say it's a modding base, but it's hard for mods to improve something that has almost no base gameplay. You can install as many mods as you like, but it will always feel like something important is missing. There isn't enough of a game in this to set a bar for quality, or a solid direction, so the mods will never fit one another too well.
It might be possible to get an okay-ish experience playing on a small server ran by someone with taste, but most servers for this are giant boring malls, overloaded with ill-fitting content, and usually just being an excuse for the admin to show off his creative mode builds.
That said, if you are planning to code something better, you should still have a copy of minetest_game at hand - just in case you need some example code for a feature that isn't too well documented.