What, you may ask, is a mega dropdown? Quite simply put a mega drop down is a large dropdown menu that lists more than just the secondary navigation items for a particular section. There are varying ways to implement this, from a full sitemap style list of all the links below the level a user is currently at, through to cleverly constructed ones that include cross promotion and imply site structure to the user.
Firstly, I should state that by and large I am a fan of mega dropdowns, but I would like to think that this is a sensible appreciation rather than a blind one. Mega dropdowns have got of bad press recently, especially from the development world. This is probably largely due to the potential SEO issues that they can present if not constructed properly, but my view is always that user experience should be the core driver for web design rather than technical opinion. That is not to say that technical opinion is not valid, far from it, and the concern that full site link listing as an approach to a mega dropdown confuses google spiders is a valid one. There are some really good blog posts out there that talk about this (for example: http://raisedbyturtles.org/mega-menus-and-seo/), where a sensible view is taken.
So why do I think they are a good thing? Primarily a good mega dropdown will be constructed to imply an element of structure whilst also serving more than one type of user. For knowledgeable users who want to deep dive into a specific area of a site, having to trawl through page after page trying to find something can be frustrating, but building a site that serves them and doesn’t offer the periphery content for less knowledgeable users conversely risks alienating potential new customers. A mega dropdown allows this journey to be constructed in a way that a user can easily choose at what point they want to drop in. In the world of fast self service this is, in my opinion, an important thing to facilitate. However, this also highlights a big drawback of mega dropdowns as well, if you want to guide a user through a specific journey then allowing them to freestyle through your site will ultimately disrupt that.