As always.....***SPOILER ALERT***
Redbeard has been mentioned so many times on Sherlock it is not worth the time and effort to go back and catalog them. We know that Redbeard is Sherlock's childhood dog that was put to sleep. Yet, there is something significant about that dog beyond just fond childhood memories and the pain of losing a pet. I have no clue what that could be, though. If that is not addressed in Series 4, I shall be greatly perturbed.
Sorry, this post was not all that profound. The Redbeard thing is really bothering me, though. DOES ANYONE HAVE ANY THEORIES AS TO WHY REDBEARD IS SO SIGNIFICANT?!??!?!
The only theory I have read online that sounds plausible is this one. If that one is true, that would be disappointing.