What for you is taboo while playing Dungeons & Dragons ? I do not want to set the tone by showing some examples so its up to you to say.
Personally or as a Dungeon Master?
Personally - excessive descriptions of gory violence or explicit sexual details. Not because I can't handle either, but because I feel it to be a waste of the playing time you have to go into details for either. Even too much detail for offline activities, if they don't add to the plot, are something I avoid.
As a Dungeon Master - outright, deliberate bullying or torture that is has no purpose in the game. Character to character interaction and conflict is normal, but for one player to make efforts to bully or take enjoyment away from other players that is not normal is seriously going to have the Wrath of Dungeon Master and possibly removal from the group. And, I guess as Dungeon Master I'd also kibosh any excessively gory or sexual details for the same reasons I won't as a player.