So this company called Ars reveled what they call Steam Gauge which is allowing them to make estimates on some public Steam data. Their technique (which samples 0.04% of estimated steam accounts a day) shows that about 37% of games owned haven't even been played once. That's pretty significant (although my own is much higher I'm sure). Have some more charts that reveal more cool stuffs:
I wonder how much this has to do with people just not having time to play them vs. people just not having time to download them. I know for me it's the latter. It takes hours of valuable internet bandwidth to download one game. That's time that can be spent collecting hats on TF2. lol.