< special effects >
Especially starting web crafters seem to be very interested in special effects. The more moving things and reactions to mouse movements the better. It may look very nice at first sight. It it will get boring very soon. Apart from that they often take a lot of bandwidth and waiting time. They attract attention, away from the rest of the page. Try to restrain yourself if you plan to use any of these gadgets.
Animated GIF's are less dangerous. But these are also not supported by every browser. They don't display them all at the same speed. The fastest may play them ten times faster than the slowest. How does your page look if only the first image of the animation is visible. Where does your attention go: towards the moving image or the rest of the page? Just take a look at the banners, that use this effect. They are noticed two to three times more than static banners.
I have seen beautiful applets, hilarious Shockwave animations, ingenious animated GIFs. But most of the time they will add very little to the impact of the page. Or worse they may be a substitute for content. A desperate attempt to hide the emptiness of the site. In the end it boils down to just one thing: CONTENT. How beautiful a site may be, if it does not contain any substantial information, it is totally useless.