An ICAO license is one issued by an ICAO contracting State and that conforms to the Standards in Annex 1.
Such a licence indicates that the holder has attained a minimum level of knowledge (and, pehaps, practical skill if it contains valid ratings) as specified in Annex 1. The problem is that Annex 1 is not very specific and this is why many States will ask for additional evidence of knowledge before permitting you to work in 'their' airspace. The control mechanism is often to require a licence issued locally and for which only certain alleviations are granted for holders of licenses issued by other States.
When you look at the international picture, rightly or wrongly, licenses issued by certain States are often considered to be of more 'value' than those issued by others. In Europe there are moves to make a license issued by any EU member State valid in all other EU States. I think all the law is in place already but, in practise, it hasn't really resulted in much movement of license holders, mainly because of language barriers.