I only have an anecdote, but: I've discovered that I have no ability to differentiate between icons while working. On my own computers, I use WMII, a minimalist tiling window manager; that is: text and no icons. Before that, I'd used "traditional" window managers that showed the icon and program name in the bottom bar (Gnome, Vista, XP, 95). Anyway, at my last job, IT decided that the computer on my desk should run Windows 7, which only has the icon in the bottom bar. It was totally unusable to me; if I was working on something and wanted to switch windows, my brain would never know which one to click; more often then not, I'd just try clicking on each of them until I got the window I wanted.
I was forced to use Windows 8.1 recently and had a similar experience - I decided to go with icons-only to see how long I could stand it before I got used to it, or reverted to icons+text. I reverted after a few days because it was really difficult to find the right button. Even icons for apps that I'd surely have seen and used many times, like Visual Studio, were much slower to find amongst the ~dozen others without the text.
The other real irritation comes from the fact that the selected button no longer has a "pushed in" effect like Windows Classic used to have, but only a very subtle colour change (and the colour varies depending on the colour of the icon too... so instead of just one "active" colour for everything, I now have to figure out what each app's active colour is to find it quickly):
