#4 Rousey might not be a fit for the current WWE era
While the WWE still manages to draw huge numbers on a weekly basis, it is no secret that the pro-wrestling culture has dramatically changed over the last 15 years. For one, the company has taken a more "PG" approach, letting go of the blood and level of violence that was seen during the "Attitude Era" of the late 90's through the late 2000's.
Most of the traditional fans are not too fond of this new trend, and this could be one reason for Rousey's WWE run could be a dud. Being a former UFC Champion caliber in mixed martial arts, fans would understandably want more raw action from Rousey, as opposed to the more controlled environment that is prevalent these days.
Even so, Rousey may still very well be at par with (or even surpass) the expectations of pro-wrestling fans. The slight issue may lie with MMA fans who would cross over to the WWE, just to see how she would do.