can I create a www like Wikipedia using Cobalt?

I do not think so. If we create special field, then perhaps. But now only using version control I do not think it is possible. I mean you can do that but not fully features wiki. I think for wiki better to use special extension.

Please tell me, what is lacks in cobalt to be a wikipedia like ?

Please tell me, what is lacks in cobalt to be a wikipedia like ?

Version control is not good enough. In wiki publics have to be able to modify articles. If you have 3 modifications you cannot merge them. If in 3 modifications only last is correct then you cannot apply only last one. Last one will include all the changes before. In other words we do not have text line based version control.