Though pandaren can be played by both Alliance and Horde, they cannot speak to each other once they've made that decision -- a decision steeped in the storyline of the Wandering Isle. However, this does mean that you can play a pandaren character no matter which side of the faction fence you happen to be on. And with the kind of history and story the pandaren have behind them, it'd be well worth looking into if you're looking for a different kind of character to roleplay with.
This is a primer of pandaren lore, and as such, it'll contain a little spoiler content. However, because of the unique nature of playable pandaren characters, we won't be divulging any major story events that take place in Pandaria. This information will mainly be starting zone info. If you're looking to avoid all Mists spoilers, however, you may want to pass on what's after the break.