My wife has no interest in the horde races either but what she like about starting characters on Cenarias:
1. We are the same level, we work on quests together, overcome challanges together, we level at the same rate,
2. Won't be ganked by a lvl 60
Those two reasons were more then enough to start a character. She is, yes another shaman and I am a warrior. So we work together quite nicely. She picked mining \ blacksmithing, myself, herbalism \ alchemy. So we aren't even competing for resources, I make potions for the two of us, she makes stuff we need . We share everything down the middle.
With Alliance, I had been playing for months before she started. She's only lvl 25 or 26 while I am 57, we have nothing in common. She needed to do the stockade, I simply solo'd it with her tagging along.
In contrast, the horde on cenarias offered a chance to be together for a little of our gaming time.