Some free games require you to download the source code and compile it yourself. Some require you to just download and install. Others are just download, unzip and run. However, by far the easiest are flash/applet web games, where you just point your browser at them and that's it - once the game has loaded you can play it straight away.
However, maybe due to this simplicity, or maybe due to the fact that a lot of web games are simple in themselves, they are not treat with the same level of respect as games that require more work to start up. It seems that the more effort you are prepared to expend to get a game working, the more time you are prepared to spend on it.
(Taking this to the extreme, I find that when I play a lot of old Speccy games, of which I have many fond memories, I give them approximately 10 seconds of my time (since that's pretty much how long it takes to load it up nowadays) whereas in the past, when it took minutes, I'd play them for longer and give them far more of a chance. Actually, I think I've just invented my own axiom: "The time a player will spend playing an average game is directly related to how long it took to them to run it.". You heard it here first!)
Anyway, back to my point. I've got a game called Island Commander, which is an applet, but is increasing in complexity. However, because of the lack of "respect" that web games get, I'm tempted to turn it into a stand-alone game so that people will take it more seriously. In addition, nothing is more galling than seeing comments by people about games written as applets, all saying how much they hate applets.
Is it mercenary to make it harder for people to play the game in the hope that they will spend more time on it? Maybe my whole premise is wrong, and flash/applet games get played far more than stand-alone games. However, out of all the most popular free open-source games out there with large communities, I can't think of any that are web games.