From Minecraft Wiki
What to expect
There is a misconception of a sort that has spread about Minecraft and Lua scripting. Notch has mentioned a little about Lua, what he wants it to be used for, and what he most certainly does not want it to be used for.
- Lua scripting will be server-side. Players will not have to download scripts within or outside of the client, and therefore no client-side modifications will be necessary.
- What does this mean for the end-users? Minecraft will still feel like Minecraft, still play like Minecraft, and essentially still be plain, old Minecraft. No fancy HUD changes, no guns, no custom objects, nothing.
- What will the server be able to do? The server CAN create custom commands, it CAN assign certain server functions to blocks, and it CAN create user groups. It most probably CAN make "premium-only servers" if Lua is open to the free-user base, which it most likely will be.
- Why will it be like this? Notch has said that he wanted Lua to be an extension for Minecraft. He does not want Lua to turn Minecraft into a game engine (such as G-Mod is to Half-Life 2). Essentially, Lua, although a nice addition, will not revolutionize game-play in Minecraft.