I've been developing websites for about ten years now, I have encountered people who love flash and those who hate it. I have and still do develop things in Flash, Ajax, PHP/MySQL and other web technologies. The issues that are being brought up by the "Adobe vs Apple" Flash Vs HTML5 is not new; it is how ever bring up issues that need to be addressed. Flash is great for many things but doing full websites is not one of them. Here is why.
Flash does not support SEO very well. Yes Adobe came out with a change in the flash player that lets spider bots crawl the flash content but they still do not rank well on search engines compared to a HTML equivalent.
The flash player is a resource hog and slows down a computer significantly. Don't believe me then bring up your process manager and surf to websites that have flash elements or are completely flash and see what happens to your processor(s) and if your on a laptop your battery.
Flash has been pretty buggy over the last few years and support for 3rd party broswers such as Firefox, Chrome and Opera seem to be lacking. Flash works just great in Internet Explorer but should be working well in other broswers and operating systems too.
Do I hate Flash? No. Then why did I just bash it there? Well eveything I said there was true but there are some things flash does do better then Ajax, CSS and other web technologies. I highly endorce the use of flash to produce this content for the web. HTML5, CSS, Ajax, and other web technologies are not great for doing things that flash is good for such as:
Banners and ads. Creating a roating banner scheme that you can modify and change really can only be done with flash. I've created a flash app that where companies can host their own banner ads and give websites they are hosting ads on a flash loader that then loads the banner ads from another server. The webmaster can then add and remove banner ads at will without having to resend the banners to the webmaster. A very handy use for flash.
Video content. Before there was h.264 we had Quicktime, Windows Media and Real Player remember that one? Loading video was painfully slow and the quality was pretty bad. Flash offered a low bandwidth solution to this problem and has worked well. For some enhanced video on the web I'm sure flash will be used but for your basic everyday just watching a video, I don't beleive flash is required. I think you will see two types of video, regular video and enhanced video content what will have some sort of interactivity that HTML5, CSS and Ajax just won't be able to do.
Games and other interactive web utilites. Like it or not flash is still the gaming platform on the web and computer based training. To write a module in PHP/MySQL, for Moddle, or another type of platform would probably take more time then to do one in flash. The flash traing modules can be easily intergrated in to Moddle, not to mention the benifits of games, video, audio that can all be integrated without having to write customized modules or plugins.
Flash is a good technology but I feel it's time on the web will soon be up (less required). As things get better with Ajax, CSS, and server side technologies, and start to do more things that flash does, flash would be considered just another technology to support. However having said that using flash for making Adobe Air apps (useful clients for cloud based applications) is an excellent niche for Flash and a growing market. With the ability to store some data locally then sysc it with the clould represents a unique affordable option for many companies to take advantage of. If Adobe really wanted flash to be openly adoped to full standardized web technology they would have to completely open source the flash player in my opinion as it is proprietary. We will be having the same discussion soon about H.264 a (video codec that is used for the web) as it is an open license for now but will it be in the future only time will tell.