The 3 biggest appeals for minecraft IMHO.
1.) Exploration
2.) Building, i.e. building large structures.
3.) Crafting and then learning how to create automated crafting setups which are self-sustaining production systems by exploiting and understanding game mechanics.
Mod and Mods packs can improve on all 3 by a huge factor which can then keep you hooked once you've gotten past the game's initial learning curve.