From: Brighton, UK
I'm going to cheat a bit here and repost an answer I just gave at the Steam forums!
The game UI is meant to be universal across desktop and mobile platforms. Notice that most buttons have text on them, and the ones that don't are quite graphical and consistent across the whole interface. For example, the icons in the lower bar, which are used as shortcuts to the agency's buildings, match the signs in the buildings themselves. These icons are quite descriptive as well (the helmet for the astronauts/cosmonauts, the headset for the flight controllers, the dollar sign for the Public Affairs Office, etc).
The other screen that makes heavy use of buttons without text is the personnel management screen, which features a bunch of icons in order to filter/sort the entries. This might seem daunting at first, but there are just a few of them. Once you figure out what they are, you no longer need to think about them.
We'll certainly consider adding tooltips if we get more reports about the UI being confusing, that's for sure, but notice that we did a conscious effort to avoid them while designing the screns. Not because we're lazy, but because we wanted the UI to be universal across all platforms.
Please let us know if you have any questions and I'll do my best to get back to you.