Why are the seas difficult to navigate at the "southern edge" of land? The question was inspired by an answer to this question on History SE, which referred to the "at times enormous waves" at the Cape of Good Hope in South Africa, and the even more "treacherous" passage around Cape Horn.

Why is the sea so hard to navigate at the southern extremities of the land masses.?Cape Horn, with a latitude in the high 50's, is easy to understand, but the Cape of Good Hope is in the 30s. Yet from what I understand, "navigability" does not extend far south of the Cape of Good Hope.

Is southern Australia and Tasmania (in the 40s latitudes) similarly hard to navigate? Would it be fore the same reasons?
Fetch.  The longer the wind has to act on the water the larger and more developed the waves become, see this site from the NOAA.  If you look at the southern portion of our planet you will note that there are few land masses to affect the wind or the wave development.  In the Wikipedia entry about prevailing winds you can see that the Southern Westerly winds are only affected by the Cape of Good Hope, Cape Horn, and to a lesser extent Australia.  Contrast this to the Northern hemisphere where North America, Europe and Asia divide the seas into smaller isolated sections.  

There are additional factors involved particularly around the capes.  If you look at the prevailing ocean currents, see global map on this site you will see there are strong currents, driven by the wind, around the southern pole region.  At the capes, these currents become more chaotic and lead to eddies.  Sometimes the eddies make the current flow against the wind.  This results in even larger and less predictable sea states.