* The ability to play the game without installing anything
Obviously you haven't played any HTML5 games. They all make you wait to download game data. And if the Internet gods have been good to you, it won't be cleared from your localstorage next time you start it up. Steam is far better.
* The ability to play the game on other peoples computers
Apples and Oranges are being compared here. The difference between Steam and web games is like the difference between video files and Youtube. Youtube is always of inferior quality, but it can play almost anywhere anytime. You don't need a windows machine with administrative rights.
* The ability to play the game without installing anything
* The ability to play the game on other peoples computers
* Cross-platform accessibility (Windows, OSX, Linux? Doesn't matter.)