We clearly document that you need a majority of nodes up to have a primary, but I can't find anything explaining our rationale for that decision. A lot of people don't fully understand why we made that choice and don't fully understand the differences/tradeoffs between a single master or multi-master system. See SERVER-9081 for one example of such confusion.

We should have a page that explains the difference between multi-master and single-master systems, explains why we chose a single-master system (strong consistency), and explains how having a single-master system necessitates the majority up requirement (network splits).