Brittney Griner is a 6'8" center for the Baylor women's basketball team who just completed her sophomore season. She has a wingspan of 7'4", can dunk the ball easily (and often does) and is most famous in some sports circles for throwing a punch. She would get eaten alive in the NBA.
While Griner has height, toughness and skill comparable to many NBA players, she weighs only 175 pounds. Griner's skill set is that of a power forward or center. The shortest NBA power forwards and centers are Griner's height, but they are substantially more built.
The Boston Celtics' Glen "Big Baby" Davis is considered an undersized power forward. Yet he is listed one inch taller than Griner and is nearly twice her weight, at 289 pounds. This is one of the smaller guys Griner would have to compete against.
Griner's greatest skill is her shot-blocking ability. As a freshman, she set the NCAA women's single-season record for blocked shots in a season. This skill would be diminished by her opponents increased height and jumping ability in the NBA.
In women's college basketball, Griner benefited from guarding players who were an entire head smaller. She'd be the small one in the NBA.
Griner doesn't have the skills to play a smaller position. In college, she played a back-to-the-basket style of offense. She didn't have the dribbling moves or range of perimeter shots to play a smaller position.
Griner is one of the best players in women's basketball. But at this point in time, for a woman to make the NBA, she has to be undeniably the best. Maya Moore is generally regarded as more talented than Griner, and would have a better shot of competing in the pros (although even she would have substantial size issues as a 6'0" small forward).
There will one day be a woman who can play basketball as well as some NBA pros, but it will take the perfect storm of size, skill and competitive older brothers.
Although Griner is an exceptionally gifted basketball player, she does not have the ability to compete with NBA players.