Command blocks. Every modder has heard about them, and dismissed them as inferior to mods. I came from command blocks to modding, and I couldn't agree more. However, they do have their power, and their power lies in their ability to do nearly anything.
Nearly wasn't enough. CommandControl ramps up the power of commands, while adding a powerful, configurable scripting syntax that gives packmakers as much power as they could ever dream of to play with the player experience.
Commands added:
/setbiome <x> <z> <biome>
/fillbiomes <x1> <z1> <x2> <z2> <biome>
/testforbiome <x> <z> [biome]
/testforbiomes <x1> <z1> <x2> <z2> <biome>
/savedata ...
/dataexecute <slice|pos|tile> <rules> <command>
/probenbt <selector> [nbt hierarchy]
/math <expression>
/dimension
/reloadcommandscripts
/man [command] [subcommand]
/motion <x> <y> <z> [selector]
All of these commands have more info to them. Use /man to find out more.
As well, <selector.objective> will expand into that player's value for that score, <selector.nbt.tag.heirarchy> will convert into that tag value.
Have fun, packmakers.