Many, perhaps most RPG tropes are actually originally from Tolkien. The Lord of the Rings and The Hobbit are basically prototypical D&D campaigns. And they may be even older than Tolkien, given how much he borrowed from even older sources.
Many of the mechanical RPG elements that are tropes/clichés today, were also created or popularized by D&D. The Class and Level System, Hit Points (arguably present in the tabletop wargames that preceded D&D, but named and popularized by D&D nonetheless), Character Alignment, the "+ 1 magic sword," and so on.
D&D is very likely the single most influential game ever, in terms of what it started and popularized, stretching even outside of the RPG genre. Its influence on video games was enormous and not to be underestimated.
Conversely, more tropes in High Fantasynovels work as codified in D&D, instead of as seen in Tolkien. You're more likely to see Magic A Is Magic A than Tolkien's deliberately mythical and mystical style, for instance.
Magic: The Gathering single-handedly invented the Collectible Card Game. Games like Yu-Gi-Oh and Chaotic owe their very existence to a man named Garfield. Yet, because of those games, Magic is sometimes dismissed as a children's card game. This despite the fact that many big-time poker players began honing their skills on Magic, and a small but significant group of people make their livings out of tournament winnings and appearance fees.
For Warhammer 40,000, their eponymous Space Marines are considered something of an unfunny by some of the fanbase. The Space Marines and their incarnations are arguably the most popular faction, and thus a lot of ire is directed at them from players of other armies. Reasons for Space Marine popularity include: the easy difficulty curve and relative forgiveness of the army book, the regularity with which the main codex is updated, being one of the 'more acceptable' armies for children to play, and in general receiving the bulk of support from Games Workshop. Thus it is that Space Marines can sometimes be considered an 'unfunny', 'scrub', or 'kiddie' army.
Don't forget that the small army size and inclusion in every starter set ever also make them among the cheapest.
The large metallic surfaces of the power armor that the space marines wear is also a lot easier to paint than the flesh and bone of many of the other armies (imperial guard, orcs, tyranid, dark eldar).
Admittedly the reason the Marines are comparatively kid-friendly is that no less than four of the other armies consider mass murder (Orks), arson (Sororitas) and rape while torturing people (Dark Eldar) or all three at the same time (Chaos) a fun way to pass the time.