D&D Fatigue

Except for a period in the 1990s, Dungeons & Dragons has always been the most popular and bestselling RPG (and, before anyone mentions Pathfinder's brief ascension during the 4e debacle, I pre-emptively respond: Pathfinder is D&D). D&D is most people's introduction to the hobby of roleplaying; it's also the only RPG that non-gamers know by name. D&D is thus the proverbial 800-lb. gorilla of roleplaying games. In a very real sense, the entirety of the hobby (and the industry supporting it) owes its very existence to D&D. 
I don't think I'm being hyperbolic when I say this. Not only is it a historical fact that roleplaying games, as we know them today, are all ultimately descended from Dungeons & Dragons, but I contend that most of them would never even exist were it not for a phenomenon I'm going to call "D&D fatigue." As I'll explain, D&D fatigue takes two distinct forms and each of them plays a vital role in the continued existence of other RPGs – and perhaps of D&D itself.

The first form of D&D fatigue comes is the more rarefied, experienced by those whose weariness with the game compels them to write their own game. For example, the second roleplaying game, Tunnels & Trolls , owes its existence to Ken St. Andre's dissatisfaction with Dungeons & Dragons, but neither St. Andre nor T&T are unique in this regard. Indeed, if you were to look at the history of RPGs, you'll quickly find many examples – many of them quite early – of people who felt that D&D was somehow lacking and resolved to improve upon it. 
The second form of D&D fatigue is more common but no less important. This is a more literal type of fatigue, one experienced by those who've simply played D&D so much that they want to play something different. In my youth, my friends and I suffered many bouts of this kind of D&D fatigue. We'd play D&D furiously for weeks or months on end, enjoying ourselves all the while, and then, at some point, one of us would look up from our copy of the Players Handbook and ask, "Do you want to play something else?" Almost invariably, we did and thank goodness for that; otherwise, we might never have had the chance to play other great RPGs like Gamma World, Traveller, or Call of Cthulhu. 
D&D fatigue is not a bad thing. As I stated at the beginning of this post, the existence of a larger roleplaying hobby depends, in large part, on people becoming dissatisfied with or tired of Dungeons & Dragons and creating and/or seeking out alternatives to it. Very few of those alternatives ever came close to rivaling D&D's popularity or sales – but they didn't have to. During the days of D&D's first faddishness, there were more than enough players to support many RPGs and the companies, large and small, that produced them. Simply by existing, D&D created a demand for alternatives that others stepped forward to fill.
Over the course of the more than four decades I've been roleplaying, I've fallen in and out of love with Dungeons & Dragons multiple times, for a variety of different reasons. In each case, I'd eventually return to playing it, because I do like D&D, warts and all. While I was feeling the full force of D&D fatigue, I'd explore other options, in the process learning to love other games I otherwise might not have noticed. I also learned to appreciate better the things I liked about D&D, so that, when I returned to playing it, I often had more fun with it than I did before. 
As I said, D&D fatigue is not a bad thing.