The Case Against Flash.
I don't know if you've noticed, but people have quite a bad reaction to Flash, in general. Sure, it can be used well, but the reaction of most visitors to something starting to load will be oh no, Flash! , followed by a hasty dash for the back button. Why is this? Well, there are a number of reasons that come together to cause it – each one, on its own, seems relatively minor, but together they make up a pretty comprehensive case against Flash.
Flash is a Plugin.
Flash isn't integrated with any web browser – instead, it's available as an installable plugin. This has a lot of downsides. The first time someone views something that users Flash, they're asked to install the Flash plugin – this takes time and is annoying, especially considering that Flash plugin isn't available for all browsers. After that, every time some Flash content appears, the Flash plugin has to be loaded into the browser before the content can even begin to be loaded, losing a vital few seconds.
Flash is Slow to Load.
Once the plugin itself has loaded, the next step is for it to load the Flash movie in question. Because Flash movies are typically so heavy in images and animation (that is, after all, the point of them), visitors will often end up spending a considerable amount of time being forced to stare at a 'loading' graphic. This is supposed to be the web, not a PlayStation – no-one wants to watch your site load.
Flash Makes Sound.
Flash upsets users because they generally have no way of knowing that it's going to make sound – many users disable all their browser's sound functions, not wanting random websites to be able to make sounds at them, but Flash sound still gets through, since it's a plugin and doesn't obey these settings. Flash is part of the reason why users end up browsing the web with their speakers turned off altogether – people just hate having unexpected sound forced on them, and they have no way of knowing whether your Flash website might suddenly start making some.
Flash is Often Unnecessary.
Because Flash lets you make little animations, many websites use it for things that are completely unnecessary and un-interactive, but that they think look 'cool'. The classic example of this is the web crime of the Flash intro: a useless piece of Flash that visitors have to sit through before they get to a website, usually saying and doing nothing useful whatsoever. Using Flash for unnecessary things is actively user-hostile, and many users have come to associate its use with that mentality.
Flash Breaks URLs.
If you let visitors navigate around within a Flash movie, that navigation isn't saved at all. If they go to another site and come back, or even just press the 'Refresh' button, they'll lose their place entirely, and have to start from the beginning again. This isn't good if they found a particular piece of information or picture – they'll be annoyed at having lost it.
Flash Breaks Right-Click.
Users like to be able to right-click, to print what they're looking at, or save it, or copy it to the clipboard – not to mention all the extra functions that they might have installed on that menu. Right-clicking on a Flash-based website, though, gives a right-click menu of things related to Flash, like whether the movie should display in high or low quality. Users just aren't interested in this menu, and are upset that they can't get their normal one back. This is an especially large problem for users that like to have more than one window open at once by using right-click followed by the 'Open in New Window' function.
Search Engines Can't Read Flash.
Finally, perhaps the most convincing argument against Flash: it's entirely invisible to search engines. Text you put in a Flash movie doesn't exist, as far as search engines are concerned. It's closed off from the rest of the web and unfindable by most of your potential visitors. That surely can't be good.