There are some replies in the FAQ section on Ethiopia at https://www.reddit.com/r/AskHistorians/wiki/africa#wiki_ethiopia . Of particular note are

• "How did Ethiopia successfully avoid colonization?" at http://www.reddit.com/r/AskHistorians/comments/qnjq9/how_did_ethiopia_successfully_avoid_colonization/ , answers by /u/snackburros and /u/commustar
• "How did Ethiopia remain independant for so long and then gain support from other colonial powers?" at https://www.reddit.com/r/AskHistorians/comments/12gn2u/how_did_ethiopia_remain_independant_for_so_long/ , answers by /u/khosikulu and ThoughtRiot1776
• "Ethiopia stayed independent during the 19th-20th C European colonization of Africa. What enabled it to stay autonomous?" at http://www.reddit.com/r/AskHistorians/comments/1ddde1/ethiopia_stayed_independent_during_the_19th20th_c/ , answers by lakerman1495 , khosikulu and guerrier_papillon

But mentioning those is intended as a starting point, not to discourage more discussion. Further questions, info, and debate are welcome.