Very little actually... one has to get to the chormosome level to distinguish much difference, other than the physical characteristics, some of which are in your question. In fact the two sub-species of Equus can interbreed, but the off sping, quite like the results of donkey/horse interbreeding, the mule are sterile. It appears, according to my book, that the zebra is closely related to the wild horse of Asia, Przhevalski's horse... an ancient breed...