This is the full list of mods that I run: http://pastebin.com/umAn8HRz
I could do a console paste, but they're freakin' huge. However, it pretty much consists of something like
[21:53:06] [Server thread/INFO]: [gregtech.common.GT_Worldgen_GT_Ore_Layer:executeWorldgen:92]: Generated Orevein: ore.mix.magnetite
Repeated with different mixes. The console correctly reports when a GUI is opened, but doesn't allow Backpacks or other inventories to be opened.
The only other mods that runs at the same time (seemingly) is ChocolateQuest, which outputs a few things like this all at once
[21:52:55] [Server thread/INFO]: [com.chocolate.chocolateQuest.entity.mob.registry.MobZombiePig:setItemColor:77]: {display:{color:6684672}}
The worldgen freeze occurs about 40 seconds after the mods that need to register text to the in-game console.