Some were good, but we found the vast majority of icon sets lacked in quality, were inconsistent, or simply too small - usually a combination of all three.
When we were building Untitled UI v1.0 we looked at dozens and dozens of open source, free, and paid icon sets. The problem is not all icon sets are created equal.
Just a quick Google search will reveal hundreds of icon sets, many of which are 100% free and open source. There is certainly no shortage of icon sets and there are many different styles of icons floating around the internet today.
When it comes to modern UI design and web design, a good quality icon set is invaluable and can make or break a design.