I love the look of drop downs, but in addition to what Robert mentions, there are two more reasons I'm in the process of dumping them completely:
I was recently redesigning a corporate website with a responsive design, and when they saw how much scrolling the drop down menu caused on smartphones, they wanted me to find another way. We went with a horizontal menu of the top hierarchy on every page, and when you click one of those pages, there's a sub menu on it with more options (the former drop down items). We styled the sub menus as links so smart phone users can just click 'em. It's actually pretty elegant and avoided the whole issue of navigational links in the sidebars. I suspect it may also distribute pagerank in a better way, since the sub-menu links are so tightly connected to the pages on which they appear.
Drop downs can also be really tough from an accessibility standpoint. If your hands shake, or you have trouble using a mouse, it can be very hard to get your cursor onto the menu item you want. I've known people who just won't stay on sites that have drop downs for this reason, and I can't blame them.
One last note: I also agree with Robert that many sites have too many navigational links. The corporate site was definitely an example of this, so we used a number of strategies to whittle it down.