Another thing I really love about the Holmes rulebook is the art, which is mix of Dave Trampier, Dave Sutherland, and Tom Wham. It's little surprise, I guess, that those three, even moreso than the great artists who came later, formed my sense of what D&D looked like. Take these two pieces by Dave Sutherland, for example:
Whatever flaws they may have -- and I am regularly told what a terrible artist Sutherland was -- there's a strange kind of groundedness to these illustrations. Look at the fighting men in these pictures. They're all wearing historical armor rather than some fantastical concoction without any basis in reality. All the fighters you see in Holmes look like this and it made a powerful impression upon me.
I'm sure military historians will be able to point out multiple problems with Sutherland's depictions here, but that's rather to miss the point. It's not about strict realism or accuracy; it's about verisimilitude. Sutherland's artwork conveys a sense that the combatants in them could have existed, even if their opponents make it clear that they didn't. I like fever dream fantasy illustrations as much as the next guy, but there's also something to be said for the kind of "meat and potatoes" pieces that Sutherland produced. They're one of the foundations on which my conception of Dungeons & Dragons is built -- much like the Holmes rulebook in which they appear.