Earlier quoted context omitted.
Flash is a threat to the openness of the web. I can't reliably watch video, play games, or do anything else requiring Flash on my computer because I run Linux. Requiring you run an OS of Adobe's choosing is antithetical to the basic tenets of the web.
No it's only antithetical to the tenets of Linux users. edit: Just saying, people who knowingly choose Linux don't expect to use Flash.
When the web relies on a technology where the "protocol" is "first install this proprietary plugin, if it's available for your setup, and if not, screw you," it is antithetical to the protocol-based approach.
Look at how much browsers have improved in the last few years, making new things possible on sites. This happened because anybody can write a browser that conforms to HTTP protocols, Javascript specifications, etc. Do you see the same kind of improvement in Flash players? No. Because there isn't competition.
The web is better off without Flash. Or anything else that can't have 100 competing implementations.