One of the challenges I often find is in licensing/copyright. Some Creative Commons attribution requirements can be a pain, for instance, rather than something that's simply MIT like my code. I don't mind a license file in a repository, but anything that says I have to include a credit line somewhere in the end product is off-putting. Some places offer free icons, but if you modify it... say, by changing the color of the icon or converting it's format, you may be violating their terms.

So I'm always up for a clearly-defined set of MIT-licensed icons.