Companies frequently moan about "re-training", but in my experience users of e.g. Word, or Excel don't need re-training, they need training. A large number of people how "Knows Word", can't use any of the feature beyond changing the font and font size, not even the "headings".

For product like Microsoft Office (or whatever it's called these days) 20% is ludicrously high. I'd guess more in the 1% - 2% range. Especially Word is way to complex for the needs of most people, Wordpad covers the needs of most home users. I also thinks that's where the recentment for the remaining features come from. It's not that there's some number of feature hiding in a corner that's the problem, it's that almost the entire application is "useless".