Why is John Gruber anti-Flash?

Why? At the core, because Flash is the only de facto web standard based on a proprietary technology. There are numerous proprietary web content plugins — including Apple’s QuickTime — but Flash is the only one that’s so ubiquitous that it’s a de facto standard. Flash is the way video is delivered over the web, and Adobe completely controls Flash. No other aspect of the web works like this. HTML, CSS, and JavaScript are all open standards, with numerous implementations, including several that are open source.

And:

Apple, with the iPhone, is solving the chicken and egg problem. For the first time ever, there is a large and growing audience of demographically desirable users who don’t have Flash installed. If you want to show video to iPhone users, you need to use H.264. …Apple isn’t trying to replace Flash with its own proprietary thing. They’re replacing it with H.264 and HTML5. This is good for everyone but Adobe.