Once you're ready to build your web site or web page you begin wondering how you are going to build it. Will it have tons of animations and interactivity? Will I fill it with graphics? How will all that affect the navigation, comprehension, accessibility and download time of the page? What are the things you need to consider when deciding which and how technologies will be used in your website?
Each website has an optimal design that fits perfectly to its thematic, requirements and visitors' type. For example, a web site offering e-cards will certainly need to catch the visitors' eyes, using images, animations and interactivity, while a site dedicated to science, teaching or investigation may not need any of those to show up their information and contents.
The amount of animation or interactivity will certainly affect the accessibility of the web page given that the technologies used to provide this type of content are usually not supported by user agents or many times not designed for people with disabilities. To consider how many people you could be letting behind I will base my study on the statistics of this site (http://www.htmlquick.com) which is completely accessible and provides a lot of textual information.
To look at some round numbers I will say that if the content of the site would be completely based on Flash or JavaScript we would be loosing from 1.77% to 4.55% of the visits due only to the lack of technologies, which could be a very large number of visitors if the site is successful. But lets take some considerations based on the most popular and problematic technologies available.
Flash
Flash is a great way to build animations and interactivity but could also be a big accessibility problem. Creating a website completely in Flash could let out a 2.78% of the visitors and make the site completely empty for search engines, which may be your primary inconvenience.
Many sites will look at Flash as a primary necessity while others may only use it for animation or won't use it at all.
JavaScript
JavaScript has similar issues to Flash. Many user agents won't support it, including search engines robots. A 1.77% of the visitors won't be able to see the information written in JavaScript nor follow the links.
Frames
Frames are nice, but should be definitively avoided. While most of the user agents support frames, search engines, as well as blind people, could be seriously confused when trying to relate the information contained in the different pages of a frameset.
Images
Images, even when not accessible, are a more benevolent alternative to Flash animations.
Some ways to generally avoid the use of conflictive technologies is to opt for more common or more supported types of information. One of the best alternatives to set the look of a website is CSS. CSS is specially designed to set presentational attributes. Use it instead of tables to set layouts.
Anyway, some sites will necessary use some or all of these technologies to display their primary content. For example, an e-cards site may need Flash to display the animated and/or interactive cards (primary content). But when the use of these technologies is not a necessity, avoiding their use to provide primary content will be good for your visitors as it will be for you.
Diego Ponce de León