It's because you've got mods that haven't been updated to work with the new tmodI keep getting an OutOfMemory exception every time I try to load a world through the updated tModLoader. It works fine in Vanilla Terraria, and I'm pretty sure I installed the new tModLoader correctly...
EDIT: Reverting back to tModLoader version 0.8.3.2 (I had the .rar on my desktop) allows it to work just fine, albeit in the non-updated form.
Read the page before thisis it working for the latest version of terraria vanilla? thx