I may have a few games that could not be installed when I first got them (bought on package deals, did not have the supported graphics card at the time, etc.), but overall I have maybe half of my library installed.
There are many games that I bought on a steam sale, tried playing 1-2 times, and uninstalled. With the Greenlight program, too many really crappy independent games are released on steam with poor quality control and bug-fixing.
The first party valve games are great, for the most part. There are also quite a few for which steam will start a third party background launcher for a game. Those can be aggravating.
I have so many games that I have not completed. At times, I will list them out and make a goal to seriously play them. Lately games have to be put on the back burner so often though.