Why does mcl like undirected graphs and why does it dislike uni-directed graphs so much?
Mathematically, the mcl iterands will be nice when the input graph is symmetric, where nice is in this case diagonally symmetric to a semi-positive definite matrix (ignore as needed). For one thing, such nice matrices can be interpreted as clusterings in a way that generalizes the interpretation of the mcl limit as a clustering (if you are curious to these intermediate clusterings, see faq entry 8.3). See the REFERENCES section for pointers to mathematical publications. The reason that mcl dislikes uni-directed graphs is not very mcl specific, it has more to do with the clustering problem itself. Somehow, directionality thwarts the notion of cluster structure. [add].