Google has (fortunately) provided some guidance - although minimal - related to Web design with the release of Instant Preview today.
The development as was mentioned here makes great Web design imperative. Some of the suggestions Google has provided are open to interpretation and will surely initiate an immense amount of discussion moving forward.
While a good user experience should always be the focus of Web design, Google may just be forcing the hand of designers to keep distractions and extraneous content at a minimum. Site simplicity and message clarity are now of the utmost important courtesy of the introduction of Instant Previews.
Google is recommending avoiding interstitial pages, ad pop-ups and other elements that interfere with content as they will affect the preview and make screenshots less attractive. This should drive every single interactive media designer, media buyers and site owner over the virtual edge. Count on work-arounds (and perhaps an increase in cloaking) being released soon.
Perhaps most important in the short term is that some videos or Flash content in previews will appear as the dreaded “puzzle piece” icon or simply a black square. While Google is working on rendering these content types, perhaps it’s time you brush up on some HTML5.
Google has attempted to make it clear that Instant Previews does not change its search algorithm or how clicks are tracked. According to Google, “If a user clicks on the title of a result and visits your site, it will count as a normal click, regardless of whether the result was previewed.” Previewing a result, however, doesn't count as a click by itself.
If you are using the nosnippet meta tag on your pages (which will cause Google to not show a text snippet in its results) you may want to remove that as the tag will also not show previews.