What makes a weld job a quality weld job?

[BlackFireN2O]

When i had posted on this forum (and others as well) about doing my own welding, most said to let the professinals handle it. But i ask, other than just being more "experienced," what makes a quality weld job quality. In other words, what makes a weld job that i would do different from a weld that a pro would do, are there any specific things that make it sturdy? Or do people just think that welding is a "black art" and should be left to the pros, so they blindly hand over their car to the "pro's." Thanks in advance for responses!