I don’t think user preferences are something that is so frequently accessed that a dedicated header button is needed, but I do think that it’s hard to know that the username opens another menu (and knowing is half the battle). We’ve also had a couple complaints about finding the log out button from time to time.
I suspect the problem is that these icons on the right are all the same size and distributed equally, and when one is pre-selected as a tab… it’s easier to understand they are tabs.

The username on the far left doesn’t share that benefit, without some interaction there’s little reason for me to think it’s anything but my static username…
An additional menu item would make it easier to find (the addition of the cog on the right)…

…and I’m not sure how critical it is to see your own username here (not at all useful for me), so I’d even go as far to do something like this…

Then the links under it could be…
Summary
Activity
Drafts
Preferences
Log out
(We already have a dedicated messages tab, so I don’t know why we’re duplicating that under the existing menu)
Curious if others have thoughts on this…