The icons are cryptic. In particular, it seems nobody can find the review icon; on Stack Overflow, where the new top bar has been tested, reviewing has declined a lot
There are probably a lot of things contributing to this in one way or another. But it's hard to ignore that, after the initial "novelty" spike, the activity in review has been declining pretty consistently ever since we changed the textual link to an icon.
Textual links (in the old design) are easier to see at a quick glance
The search box is unnecessarily huge now and changes size in a disturbing way
The numbers on the messages and achievements icons are smaller and therefore harder to read, particularly if you're trying to optimise resolution on a small laptop screen
The thing I like the new design is a sort of representation of navigation. The user profile icon is near the navigation button just as the user translating in the different StackExchange networks. The StackExchange icon is along with the search to navigate within the stack.