The basic topics of the letter cover:
It's not open.
The "full web.
Reliability, security and performance.
One of the web's greatest hidden evils: rollovers. Rollover flash ads don't fit with the Touch interface.
Basically, Flash is built around the idea of mouse input, and would need to be "rewritten" to work well on touch devices.
If developers grow dependent on third party development libraries and tools, they can only take advantage of platform enhancements if and when the third party chooses to adopt the new features."
I'd like to add my own two cents about the PC side concerning Adobe Flash:
While I have worked for many PC shops over the years to do their Mac work ... occasionally I'm asked to assist with the influx of PCs that need to be virus cleaned. The main culprits for loading viruses on PCs are Flash and infected Adobe PDFs.
And then the Mac side of things:
Since I sell my own networking device for Macs - one common question I get is how to speed up the internet. My first suggestion is always to try to avoid sites with Flash ... but if you must visit them ... download a program called Click To Flash.
From The Click To Flash description:
The advantages of ClickToFlash are numerous. Since Flash isn't loaded until you specifically ask for it, your CPU usage will stay at normal levels when browsing the web. This has tons of benefits: web browsing stays speedy, your Mac laptop won't get as hot, and your Mac's fan won't come on as often. In fact, we guarantee* that ClickToFlash will quintuple your battery life and that it will protect those precious parts of your body on which you rest your laptop!
Best of all, ClickToFlash even supports viewing all those ADORABLE meowing cat videos, annoying dog videos, and hilarious rickrolls from YouTube without using Flash at all! That's because YouTube also offers H.264 videos, which are used when viewing YouTube on the iPhone. With ClickToFlash, you get access to those same, higher quality videos.