That you can make it exactly what you want to. If there isn't a vanilla feature, texturepack, or mod that has what you want, you can always make it yourself. Or maybe it's the friends we made along the way.

The fact that it has soo many mods/modpacks to try. And it's very fun.

meeting new people

The fact that Minecraft is the best example that you shouldn't judge a game by its looks alone (or a book by its cover). Because this game runs so much deeper than many people give it credit for... True story: when my gf first showed me the game on her laptop I was a little "so so" about it, I heard much about it and it looked kinda fun but... nah, I dunno.

So then she sat me behind a demo version in creative and showed me some very basic redstone options of which she was convinced that I'd like them. One hour or so later and I made a rather fun "shift register" which worked decently well. I mean, OMG... that's basic digital logic circuitry, the very basis which powers PC's! Simple game indeed, NOT :) Needless to say I've become quite a fan of redstone over the years. Then we got command blocks, and their variations and nowadays even whole datapacks!

And don't even get me started on all the amazing things you can do and discover with modded Minecraft (but... more about that soonish; I'm preparing a blogpost: a "redux" of one I made a year ago).

My favorite feature of Minecraft has to be the customizability. With the power of mods, datapacks, and add-ons, you can customize Minecraft all you want. Even without mods, Minecraft is still highly customizable with the many world types, gamemodes, and the brand new updates.

I love the support (from Mojang) for datapacking and custom world generation that we've been getting. They add a little more every update!

I love meeting new people on my server. One of the best things. Especially if they share the love for the game like we do.