The significance of characters having initial D in their name in One Piece At the first, I think, initial D is the fabulous initial. Until now, all of D are the strongest people in One Piece, and that been proven.

According to me, whose people have initial D is the good man. But why Marshall D. Teach is the bad man?
Elaborating a bit on the will of D. In short all people born with the middle initial of D have the power to greatly influence or change the current course of history. No person with the name "D." has ever been just a normal person they are the people you see in the news headlines traveling the world and wrecking s***. 

Ace, Luffy, Blackbeard, Dragon, Trafalager Law and Gol D. Rodgers all bear this name and are all major characters in the series. 

Now determining who is a a "good guy" and who is a "bad guy" is not based off of having the name. It only means that these characters are major and significant or will become major and significant later. 

There is some speculation as to the will of D relating to the poneglyphs and the tenryubito in the holy land and whatnot but no cannon plot has focused on that yet. My suggestion is to just stay tuned...