I kinda care about 40K fluff. I would say that the Tau stuff was interesting as an idea, but in the end they seemed to be a way in game terms to reintroduce Squats, without having space Dwarves (AKA the plasma techy race).
Necrons are an odd one. Once again it was a "We need to add a new thing to sell" moment, and the original release on a White Dwarf with a ballista pack attached was a first time they had ever done that. Though the rules for the model were insane. He had marine style armour and stats and every time he died he got back up with "I'll be back" on a 2+ unmodifiable, to top that he cost the same as a marine I think. The later releases of Necrons toned them down a lot, what left people upset that there nigh invulnerable warriors were now not as tough as before. But they did get a load of bonus units and stuff to add to their ranks rather than just Immortals, Warriors, Destroyers and Lord.
Retconned history is always an annoyance, but more so in my opinion is with Forge Worlds HH game when coming in to 40K. This shows that each legion / chapter has its own specialist units that are armoured in a different way and have different abilities and skills compared to others. Of course the SM codex has chapter traits as a partial thing like that and very old Chaos Codex 4th edition had each chapter having this own skill, but were did the:
Emps Children Phalanx Termys go and Palatine Blades (with some disorder having had the legion split up might have taken a bit of a toll, but you would still expect some people to be judged at the elites)
Death Guard Grave Warden and Deathshroud (technically also Deathshroud are all dead, but surely the idea of having Termys with Power Scythes lives on).
Iron Hand Gorgons and Immortals...
Of course things like Emps Children Kakophoni became Noise Marines and Word Eater Rampagers and Butchers became Berserkers. But it seems like so many other things have been lost for what appears to be little to no reason.
Personally I would LOVE to see FW bring out 40K rules for some of these units that even if not identical to the 30K versions, but at least similar enough to the original to keep the spirit of them. I wouldn't mind even having to pay a premium cost for some of these units. Of course for competitive play that would make them unusable, but since when have units like Mutilators or Warp Talons been worth taking.
It also annoys me as some of those above with things like the Black Crusade that clearly killed off several characters, only to have them reappear in the next book like nothing happened. If you are progressing the story, you shouldn't just say "Well that didn't happen yet in this edition." Surely with all the new units and vehicles that have been created, then the battle that people like Eldrad died in might not have happened because now he has a bunch of Wraithknights and stuff backing him up. This could have meant the games that were played and reported via the website to influence the results would not have played out as they did. At least in the Blood Angels book that came after some fight with some Orks I am sure I remember that they did put something in their to say "Tycho is dead, but you can still use him in historic battles."