Well in basic minecraft it's the architecture, the redstone "magic" and exploring when you ad mods and mod packs like technic/tekkit it's still the architecture and then even more complex redstone "magic" and engineering.
I first bought the bought the game in late alpha I think since it was just before the Halloween update but that account got hacked so I bought it again in semi-late beta between my account being hacked and me buying minecraft again I played using a hacked client.