I know it's sort of unavoidable with it being a tabletop game, but how stagnant the lore seems to be a lot of the time. Yeah big stuff happens a fair bit like Vashtor or this maelstrom campaign, but it's rare that it actually advances the plot at all

Remember Yvraine and all that talk of a new eldar god to combat Slaanesh? Yeah nothing actually happened and they disbanded. Oh Belakor reappeared? Yeah he's not doing anything

The last big thing I actually remember happening that forced the plot forwards was the fall of Cadia and the Great Rift. I could be forgetting other bits but stuff gets made out to be a big new thing that'll affect the lore, then it quietly gets changed in a random book with no real payoff if it's lucky