The authentication-related stuff also is broken in most (old) versions, same goes with skins. These can be fixed through mods, though I'm not really sure if it's worth the effort.

TrueCraft seems pretty interesting. I'm always a fan of open source games, and voxel games interest me a bit more than other types of games.

MineTest had/has some potential, though it really needs some polishing. Movement doesn't feel as smooth as in most other games, the textures aren't the best, same goes with the menus. It has a decent modding API, though it's mostly server-based which means there aren't any client-side mods (that I know of).

Terasology is also an interesting open source voxel game, though the overall aesthetic of the game doesn't feel consistent. It has simple Minecraft-like textures, yet fancy effects/shaders. I'm also not a fan of the menus, though that's just me being picky.

I think what makes Minecraft a bit different from these open-source games is how everything is laid out. Movement feels smooth, menus are consistent and simple, the font is legible, the controls are fine, the textures are easy to get used to and the sounds/music are great.

I'm interested in seeing how TrueCraft ends up, if it will be following the style of Minecraft in terms of visuals and gameplay.