I think it's important to distinguish between having some knowledge of the thing you're critiquing (which is what I think Jauqo is getting at) versus possessing a specific set of skills.
Example:
If I tried to critique a dancer, they could rightly say "Well, let's see what you got". Well, I got nothin'; I don't have the skill set so I would look a fool, and the critiqued dancer could feel justified in dismissing my critique. But what's really happening is that I don't know jack about what I am critiquing, and that is the real reason why my critique would lack value.
As opposed to:
Me critiquing a saxophonist. He could also say to me "Well, let's see what you got". In this case too since I lack the skills to play the sax I would suck and he could feel equally confident dismissing my critique. Initially it would appear my critique of his playing was just as hollow as my dancing critique.
The difference is I know the subject matter here. If he's out of tune, has bad time, poor note choices, uneven tone, etc, etc...I do not need the specific skill set of playing his axe to be vaild in my critique. Rather, I need to know what I'm talking about.
You don't have to be an amazing bassist to be able to make a valid critique of another bassist, but you do need to know what you're talking about is you wish your critique to carry any weight.
That's how I see it anyway