Blizzard has posted a pretty thorough "how to" in regards to achievements in World of Warcraft. If they are like most other games, then people are trying their darnedest to get the most. How do you obtain many of them and what rewards will you receive?

Completing reputation achievements can be time consuming, but the odds are good that you've already completed some of them retroactively. Practically all reputation achievements can be completed by increasing your reputation with various factions, so if you're already exalted with some factions, you will automatically unlock the associated achievements. Completing unfinished reputation achievements can reveal cool aspects about the world that you might have missed the previous time you played through, so it's definitely worth spending the time.

Read the entire guide over at the official WoW site