"There's no visual cues to let me know that there's sub menus"
It is a fairly common pattern in nav menus to have an indicator -- the unread count or down arrow in this case -- at the top level, but not at the second or third level, and instead rely on simply hovering on the submenu to open the next level.
"there's no visual cues to let me know the buttons are clickable until I am already interacting with them"
Except one of the most powerful cues, action verbs in button copy. You could easily make the same claim you just made about say, menu item styles in OSX. The reason why the buttons seem so bad in this case is in part because it's just a demo with a wall of buttons.
Where I agree with people critical of flat designs, such as what Nielsen says, is that it breaks down in anything with anything particular complicated or with many menus and actions. That clutter of meaningless buttons at the top of the demo is a good example. But for cases where you have a simple design that can be driven by color and typography, it can work. Have a little imagination. There is no 100% good or bad trend in design. Just different solutions for different problems.