Mobs, crafting, circuitry, day/night cycle (unless Minetest has that now), the nether, crafting up to "higher tech" from specific mob drops, farming, I think fishing actually came in pretty early, boats, everything you mine can be used in various crafting recipes, some unique to the substance, standard work tool set (shovel, pick, hoe, hatchet, sword, etc etc) tools like compasses, clocks, I must list again redstone stuff (you could build lots of cool stuff in a fairly small circuitry basement, even without repeaters). In general it's just…game! Challenges, rewards, actually decent procedural generation (reminder, I'm talking about late alpha/early beta), and enough control doodads to build cool stuff. I can't remember when pistons/sticky pistons got put in, but with that, powered doors and trapdoors, ladders holding out water, tricks with torches and falling blocks, so on, so on…it was fun seeing what you could make.
Stuff like lava and falling blocks I never saw in minetest. Maybe it has it now. When I played it, there were really no game physics at all. In Minecraft, even the weird glitchy water and lava spread and flow system was fun and useful to make things with once you got used to it.
I ran a server and most of us agreed the new health system was stupid. Also, Bukkit was cool for a while, but running a server with hMod and plugins for it before it stopped being maintained was really the peak MC experience for me. I didn't do permissions systems or anything, just had some cool utility mods and would sometimes enable the crazy cheaty stuff like CraftBook at the pleas of my users, just to see what hijinks they pulled. Really it was mainly giving everyone a /home (and later limited personal warps), /tpa to request teles to people or to tele people to them, and eventually general and private warps. I thought through whether or not I wanted to add any teleportation at all, and added it slowly and in pieces. It was a cool time.
Then, of course you had to have Movecraft, even on a survival server, when it got stable. I mean how could you not? It would be anti-fun to not add it.
A lot of Holla Forums will never realize it because they were busy being hipsters not liking thing, but before Minecraft was the "viral sensation" that your little brothers play and is full of content but a trash experience now with community autism to rival Homestuck, it was a really cool and unique and somewhat punishing game college kids played and built cool stuff in. (obviously, any survival server should be set to Hard…the only person who complained about that, hate to say it, was the one generic whiny liberal in our circle of friends)