When designing this years site, I tried hard to make a site viewable to absolutely everyone and anyone while still making it look decent. Our site uses HTML and CSS and thats it. Similar to Patrick, all our scripting is done server-side (PHP).
Thanks to CSS, you can do a lot of things to improve your site that don't prevent people from viewing it. Flash takes longer to load, is a pain to update, and you'll lose part of your audience, JavaScript loses a big audience, especially with a disabled browser.
Next year I plan to start processing headers and then depending on browser/what they can accept, I'll show different things.
Long Story Short: I felt the priority should be conveying a lot of information to anyone who wants it, in a media that I can easily update (I use VI to edit it, Lynx to preview it
).