Here's a laundry list of suggestions:

• Add image and raw BBCode support to the forums

• Remove The Impossible Game 2 section, it's just a link to another site anyways

• Allow users to browse a creator's levels, and automatically add missing levels upon clicking on one that hasn't been added yet

• Make the forums more accessible, with just one click of a button on the header

• Site themes

• Add descriptions for tags

• Level genres

• Make the reputation system clearer

• Personal user titles for the forums

• More forum sections

• Friend and following system

• Quote other users on the forum