The following diagram shows the differences between “round” and “broken” fonts. It is obvious that documents printed in “old” fonts look very different and that they are hard to read, even for humans.
Image Source: http://de.wikipedia.org/wiki/Gebrochene_Schrift
More about this topic can be found on Blackletter Fonts on Wikipedia
Back to: Historic OCR Overview
A more technical details about the optical character recognition (OCR) can be found on the ABBYY Developer Portal