Ignore the retarded samefagging OP shill above, he doesn't know jack shit about what he's talking about
>Why is there so little visual difference between 30K and 40K
The main reason is John Blanche being the main Horus Heresy concept artist in 2002-2003. He mixed the art of the card game and Index Astartes and added his trademark sepia gothic style to it. Then in 2004, Black Library's Visions of Heresy became the series bible that the novels and game are based on.

In insight, Blanche's art was anachronic and didn't properly match the idea of the early 30k Imperium being a golden age, so later artists deviated from it (like Neil Robert's Prospero being closer to the original card game than Blanche's version). But the "damage" was already done