I vote for having it be a customizable user preference. I personally want to be able to just click (i.e. left-click) the System Tray icon to be able to launch Evernote, but I definitely see the functionality of having the 'Quick Note' be the default. For Evernote developers to change this major functionality, you can bet many, many folks asked for it.
People are confusing "System Tray" and "Taskbar". Technically, they are considered two different areas even though the System Tray lives on the Taskbar. If you see your Taskbar horizontally at the bottom of your screen, the System Tray is located all the way to the right (I have my Taskbar vertically on the right of my display so my System Tray is at the bottom of my Taskbar). The name of this thread unfortunately is "Taskbar Actions" which is technically wrong. What is being discussed, I believe, is the action when one clicks on the Evernote icon in the System Tray, not the Taskbar.
PRO-TIP: If you pin Evernote to your Taskbar, then it does indeed open by just clicking on it (i.e. left-click). To pin it, all you have to do is click the Windows button and scroll down to Evernote in the Apps list and drag the Evernote icon onto the Taskbar (not into the System Tray). Voila! It's an optional workaround for the folks who want to just click an icon to start Evernote. At best, you can have the best of both worlds. At worst, it's a viable work-around until (and if) the Evernote developers give us the option to configure the System Tray's icon behavior.