Explained by Sven Lennartz, co-founder of Smashing Magazine:

Flash is powerful. Using it properly, you can create stunning visual experience and offer your visitors incredible user interaction. [...] Flash designers can achieve results which simply aren’t possible with (X)HTML and CSS. Whether you like Flash or not, it empowers the Web with functionalities which make it an incredibly interactive medium.

Flash-based web-sites can look not like web-sites at all; Flash-designers tend to use colorful, dynamic, loud and extremely vibrant approaches, with effects and animation, music and videos, stunning visual elements and interactive navigation menus.