My bike came with a standard MTB style flat bar but the position is far from neutral and resulted in wrist and shoulder pain. I switched to On-One Mary for a more swoopy design and that solved the problem.
To see what "neutral" looks like in your wrists is not simple. You can easily hold your hands out and say "look no bend" but you really need to study biomechanics or some practical application such as martial arts to fully understand the issue.
There are plenty of good example of grip shape and geometry from tools (inc stone age tools), weapons, military/ergonomic studies etc but plenty of consumer goods still come with crappy grips.