I know his work gets memed on a bit by the community but he really was imaginative. A visionary whose vision hasnt been followed much since. Dan Abnett dedicated "The End and the Death" to him. It was nice to see his work recently bein g re-embraced by GW. I once saw a story online, which was possibly apocryphal, that his daughter once took shelter in a GW during some rain. She mentioned to the staff that her dad had written a GW novel. When they found out who he was, she was asked to leave. No idea of the truth of that story but it shows how he was once viewed.
I didnt read Space Marine until recently, but I read the first two inquisition books as a kid. The bits that stood out to me were the Mechancicus runaway who didnt want to be a servitor, or the scene where the inqusitor has to figure out how to torture a slaaneshi masochist. He does so by relieving them of pain by cutting away nipple rings embedded in their flesh.
I didnt know till after he had passed that he wrote the script for AI - the Speilberg film that would have been Kubrick's last film had he not died during its making. Wikipedia says "Spielberg remained close to Watson's treatment for the screenplay and dedicated the film to Kubrick."