The problem with Flash is that it is a plugin, and not part of the browser. Being a plugin, flash has no real way of communicating with the browser (yes, an extension can, but a plugin is not an extension). So the browser leaves a space for the plugin, letting it do what it wants, and shares exclusive focus locks with the plugin. This means you are either in Flash's context, or the browser's context. So by design, flash can't do anything but grab all the keyboard commands.

Then we get to html5. This is a part of the browser. The browser can suspend execution, ensure gestures work before the program responds or any other kind of integration.

As for mouse vs touch screen, I would find a flash site designed for a mouse quite annoying to use with a touch screen. Mouse environments are designed with elements close together, so you don't need to drag your mouse far. You can also use 'hover' effects, where your mouse hovers over an element. This is opposed to a touch screen, where I can easily move my hand/finger across the screen and can't 'hover' over elements. A touch screen also requires bigger hit targets, as you have less accuracy than a mouse.

I wholeheartedly agree that applications which use the idea of hovering will not work well on a tablet. That is not a flaw that's inherent of Flash, though. It's as true of Flash as it is of HTML5/js and native applications. Hovering can therefore not be the reason why Flash does not work well with tablets, because if it were, the other approaches would work just as badly.

Any reason for Flash not to work on touch based devices must be not also be applicable to other environments, like HTML5 or native, nor can such reasons be valid if they also apply to different kinds of hardware, for example desktop computers.

I don't have a tablet, nor access to one, so I hadn't thought of the swiping feature that was mentioned in another comment in this thread, which is one such reason. Flash's jail-like environment precludes cross-boundary swiping. Swiping is a feature that's almost unique to touch-based devices. Mouse gestures come close, but I don't believe they're widely used. The problems with it are also unique to browser plugins like Flash, because it does not exist in HTML5-based applications, nor native ones.

I'm having a hard time accepting that is the only reason, though. Is swiping really that important that it and it alone is enough to doom Flash and browser plugins in general?

Most flash apps today heavily rely on hover-over events. Try using any current flash app without sending hover-over events -- it's difficult. On youtube, you the volume control and time scrubber are hidden from view until you hover over them.

This problem also exists for web apps heavily javascript driven, but seems to be less of a problem compared to flash apps. It's probably because flash apps are constrained in a small space so the interface needs to be more compact & hover driven.

Flash developers usually get a fixed chunk of space to work with and any trick they can use to squeeze 'more UI' out of that same space will be used. If hovering is no longer an option you can expect the size of these blocks to increase to offset the extra real estate required to accommodate all the bits and pieces of the UI.

What is more likely is that the user agent will be detected and that if it is determined that the user has no mouse that either some fall back mode will be used or that there will be an alternative UI preserving the basics will be presented, or a UI that will use the first 'tap' on the screen as a substitute for a hover.

That's not inherent to Flash, it's just an idiosyncrasy of Flash developers. It can and probably will change.

I can imagine similar issues, like gestures that span both flash and web.