Infinite Scale of the Sands Rep

Step by Step Guide

1. Kill Archimonde (last boss)
2. Head back to the Horde Base
3. Kill the adds (12 rep each)
4. Head towards the world tree area (where you killed archimonde)
5. When the name changes to Nordrassil, head back to the Horde Camp.
6. Repeat steps 3-5 and keep going until you hit exalted. 

This is useful not only for rep, but for recipes and epic drops as well.

Here's a screenie.