With plot armour I mean a character/squad/army/race being protected by the "story", even thought reality would obviously say other wise. For example a certain Ultramarine Captain named Uriel Ventris has a MASSIVE plot armour. He threathened the Nightbringer with a MELTA BOMB, escaped from a Daemon world, etc.
In other words by all logic the Tau should be dead already, however the story says otherwise. They always survive, usually due to some odd reason. For example they defeated a Tyranid Hive Fleet, because it fell ASLEEP after a Warp jump (what the hell??).
Also the Necrons used to be one of the major threats alongside Tyranids and Orks with the "likely" endgame. However now the Necrons have been LOLNERF'D and they have become basically useless. Most of the Necrons that had not awakened yet from Stasis are pretty much destroyed now. Necrons can no longer repair Plasma damage. Most Necron Lords have god complexes and are not all that faithful to the C'tan. Also the Void Dragon, who used to be Necron's endgame (Void Dragon and the rest of the Necrons were supposed to wake up and kill everything) has now been changed to being LOLPWND by the Emperor and now is forever locked up in Mars.