Created attachment 5854 shows the flow of the mouse to use the menu In my opinion the structure of the main menu is confusing for users. I'll reference the numbers in my attached image as (1), (2) and (3). It's extremely uncommon for menus that an 'outer part' (2) to change 'inner parts' (3). In the western world information and menus expand and flow from the top left to the bottom right. That's how you read newspapers, books and websites. Menus should and mostly do follow this concept. Some desktops place the panel at the bottom, the menu expands upwards - everybody has seen this. But to have to go to the left, from (2) to (3), is very uncommon and counter-intuitive. I got aware watching a relative trying - and failing - to use the menu. She just didn't try clicking on the right side (2), all she tried was the left side (3). For the ones knowing the usability book "Don't make me think" (http://sensible.com/dmmt.html): This menu layout makes you think.
There is already an option to reverse the side the categories are on ("Position categories next to panel button"). I've never been happy with the phrasing of the option, and I am open to suggestions about a better way to word it. I disagree that this is an uncommon menu layout, as it very closely resembles the Windows Vista, Windows 7, and upcoming Windows 10 start menus. The reason Whisker Menu is (by default) laid out in this manner is to allow the fastest access to your favorites and search.
As you can already change which side the categories are shown on, and the purpose is to make it faster to reach favorites or search, I will not change the default layout. Sorry.