High wool content with no slither is a must, as is a properly fitting hood, though in my experience of cloaks in all winds and weathers (including horrific horizontal rainstorms) hood control tends to sort itself out when the fabric is weighty enough. A really good cloak gets slightly heavier in the wet, without getting too cumbersome or without getting you wet underneath.

The only time I suffered severe cloak embarassment was when I wore one of white velvet with red trimmings. The rain acted pretty much like a washing machine with very pink consequences.