Should the covers of RPG core books be designed to serve a purpose? As a thought experiment, I propose that core book cover art should be designed to answer two questions: who are the characters and what do they do?
(I also want the art to be good, but that goes without saying.)
Let's see how a bunch of RPG covers stack up according to my rubric:
Moldvay Basic D&D
The gold standard of D&D covers. The game is called Dungeons & Dragons, the art shows that the player characters are fantasy adventurers who go into dungeons and encounter monsters--like dragons, for instance. Easy win.
3.5 Dungeons & Dragons
Absolute failure. What is this game about? Also: ugly and so very brown. If anything, the only vibe it gives off is about what the player, not the character, will have to do in this game: it might be a warning saying "This is a textbook, get ready to study."
Cover art is a place where Pathfinder smokes 3.5 D&D. Borrowing a page from Moldvay, we get fantasy adventurers versus fantasy beasts in fantasy dungeons. It does what it says on the tin.
4e Dungeons & Dragons
We get a sense of who the player characters are on this cover, but not of what they do. You could argue that it looks like they're exploring subterranean depths, but it looks way too cautious for the big set-piece battle style that 4e supports.
5e Dungeons & Dragons
A return to form for D&D cover art: adventurers fighting monsters in fantasy locales. The slightly over-the-top heroics of the cover compared to Moldvay Basic give a fairly good indication that D&D isn't a necessarily a meat grinder anymore.
1e Warhammer Fantasy Roleplay
The characters depicted on the cover look like they were rolled off of random tables, stuck together in a party, and are now facing a tough battle that will leave some of them dead and others hideously wounded. I'd say that's a good encapsulation of the WRFP experience, actually.
7e Call of Cthulhu
It is usually the case that earlier editions of Call of Cthulhu have moody, evocative covers that show Cthulhu being generally menacing and whatnot, which really doesn't give an indication of what the game is about. The newest edition does a better job of this by splitting the game into two books, and thus has two covers. The cover of player's guide shows investigators getting way over their heads by discovering a cult idol--a pretty good indication of how the game is likely to play out.
I love the Brom painting on the cover of this book, but...you're probably not going to get to play a harrowed gunslinger so it feels a bit like false advertising.
The cover reads as "fantasy Flock of Seagulls steal data and do violent stuff in a cyberpunk dystopia," so it nails the premise fairly well. It may also hint at the idea of the party's hacker going on their own Matrix adventure while everybody else does other stuff.
So, we play the alien-chicks version of Charlie's Angels trapped in that one Rick Springfield video? No? Oh...in that case, this cover doesn't work for me; it really doesn't tell me anything about the player characters or what they get up to. Also, I know this is a beloved piece of RPG art, but I've always found it really static and a bit ugly.
Savage Worlds RIFTS
Okay, the amount of trade dress on the cover is excessive, but this gives me the impression of playing a post-apocalyptic punk with crazy weapons who fights robot Nazis, which is actually much closer in spirit to what RIFTS is about than the original cover.
Yeah, I would have no idea what this is about if I saw it on a shelf. Ridiculously uninformative. The cover is actually really attractive, especially when you see it in person, but it doesn't get me excited about the premise of the game itself.
Swords & Wizardry Complete
I'm not going to go to hard on this one since it's already been a lightning rod for grognard ire, but it suffers from the same problems as the Burning Wheel cover above. While I really like the image, it doesn't really indicate that the game is an OD&D clone.
Seriously, go fuck yourself.