I bought the game at the end of Sept 2010, after a friend showed it to me As for what kept me playing, an older player, even then,, I loved the sandbox "infinite" feel of it. By making a crafting bench you could create even more things a just endlessly travel on Java worlds. Back in Alpha, there was nothing like it when you entered a more snowy area and when the snow did start to fall, slowly the chimes of the classic c-18 music came in, especially Sweden.
After half-a-dozen worlds through Alpha/Beta, ironically it was the 2nd saved world I kept that became my long-term world today, still played consistantly 15 years after it's creation and perhaps one of the longest running survival worlds out there. I don't reguard this as a "forever world" as that's a modern term and this world existed before the term, a modern term, was even thought of. I prefer "long-term world" for mine. It's not a city or town either it's a continually developing world and coming up with new ways to expand the world through building and add lore is also what keeps me invested and playing.