It's been usable by the average user for a long time now, but first the average user needs to get it. Average users usually need a lot of help to obtain operating systems that are not sold in boxes or that don't come preinstalled. That and the selection of compatible hardware, which has not always been easy. Luckily, most hardware makers are a little better at building compatible computers these days. Worst: average users have a lot to unlearn.
My mom (she's 78) is on Linux since 2006 or so. Moved from Windows to Mac and then to Linux. Never had an issue.
The key is what the user expects. If you approach Linux wanting a seamless transition from Windows, you'll be frustrated. Everything makes sense, but it's different. Your software won't run. If you approach Linux without expecting it to be like Windows, you'll be happy. The average user overestimates how much they know about computing and bring their own baggage. That baggage holds them down. They want to download Nero to burn a CD. They want to download Firefox from the firefox.org website. The whole idea of a package manager is completely alien to the average user (or was, until the advent of the app store).