Many documents can be short quick ideas, for example a blog post referencing other readings. Ponder makes it easy to gather ideas from any resource you find online, and while stars and search can help you dig one up that you remember, it also has a mechanism for identifying longer form writing called "long reads".
Ponder does count the word length of the documents you have annotated, but our "long read" flag is also based off of your Ponder group mates having spent time reading it as well.
The black bar at the top indicates a long read, and like all nuggets, the exact word count and reading time statistics are described in the summary above the document title.
The long read criteria is currently fairly simple: documents that someone has spent at least five minutes reading and are of at least 1500 words long are tagged.