I enjoy overly specific threads about minutia to the point where it is essentially parody.
On another forum I once made a thread to discuss all the characters in video games that have too many arms (think Mortal Kombat Kintaro). Later I made a thread about the proliferation of blob-based characters in games (de Blob, A Boy and his Blob etc.).
I always wanted to make a thread about posting pictures with one dominant color, and do one color at a time every week, so that the title would be the color all in caps, and it would rotate the language used for the title so when I get to black it would be in Spanish.