Appropriate Technology
The web is a complicated ugly mess. There are lots of fancy tricks like javascript, flash, and VRML. Any one of them will fail for a significant number of people. We're not just talking about people with old computers; We're talking about people browsing on mobile phones; We're talking about people using computers at work where the IT department won't allow such things; We're talking about people in libraries and internet cafés; We're talking about ordinary people who are concerned about security.
If you take care, you can make the page gracefully degrade so that users without the fancy technology still get the information, but there is always a danger that the disadvantaged users will become second class visitors.
Perhaps your site doesn't need those visitors. Go ahead, It's your site.
Perhaps you want to do something that can't be done without those technologies. Fine, but don't think that everyone will turn on javascript or download plugins just to see your page. Always ask "is this technique really necessary?"