BtB are three grouped letters that mean very little to me anymore. It's like asking someone what they thought of a debate, and their response is "Interesting." How? Why? Good or Bad? Who knows!
'BtB' is incredibly vague, and doesn't describe quality of roleplay as a writing style, authenticity for the genre its based in, visual aesthetic for the build you can expect, or even a marker for the type of players you might meet.
It's being used as an adjective for something that has taken on a whole new meaning. Rather than say "I play in a role play genre so authentically true to its lore that it's 'by the books,'" it feels like its come to mean "In this moment I will stop acknowledging my typist and become one with my character, citing quotations without analyzing them for meaning or greater understanding."
Maybe you're not a RL Gorean lifestyler, but you play one in Second Life...
"The difference between reality and fiction? Fiction has to make sense." Tom Clancy