Is it really true that there is no standardized way to create dropdown menus for a main navigation? This seems like a critical function, if missing. I can think of many functions currently available in Webflow that seem much lower on the priority list.
Found a few work-arounds here:
The methods are hackalicious at best. An easier way to hack it would be to create multiple navigations (each containing all of the contents to be included in the dropdown) and then set each to "Menu Icon For: All devices" You can then custom style each of these menu icons and set it to "dropdown". Example: http://dropdowntest.webflow.com/ (of course this causes issues when you really need to switch to a mobile nav on smaller devices)
Ultimately, the functionality is there, just not in a good way for ease of creation. Adobe Muse has a pretty good method built in using the site plan as the structure for the menu and submenus... but of course their code output is useless.