Question: I want to buy the full version of Minecraft, but I'm one of those creative types. What are some of the differences in Creative mode in the full version of Minecraft?

Answer: 
From [the wiki](http://www.minecraftwiki.net/wiki/Classic#Advantages_of_purchasing) about the classic version:

> 
> Advantages of purchasing
> 
> 
> Although this mode is free to the public, there are several advantages
>  made available to those who have purchased the game. Some of them are
>  the ability to:  
> 
>  - Use custom skins in both singleplayer and multiplayer.  
> 
>  - Use mods that require placement of files in the original minecraft.jar folder.  
> 
>  - **Build with more blocks and items.**  
> 
>  - Use crafting and create items.  
> 
>  - Combat mobs. 
> 
> 
> 

From a purely creative point of view, you will get more space, and more blocks to choose from, in the full version.

Keep in mind also that Minecraft classic is a prototype which has not been updated since 2009. From a technical point of view, the new version (full) will perform better, and look better (new lighting system).
You can play the free version, so I won't waste too many words about it. You have a small sandbox and a few kinds of blocks you can mess around and build with. You can explore the randomly generated world for caves, pockets of air, etc.; you can grief with water sources... It's the core of Minecraft.

The paid version features a world many times as large, with blocks that do actually something rather than just sitting there (with the exception of the sponge, the only "useful" block in the "free" version of Minecraft and is completely useless in the paid version\*).

There's also adventure mode, which lets people play through your map without their ability to modify it in ways that break it.

\*As of version 1.8, sponges soak up water in a 5x5 radius, similar to Miencraft Classic