How's that for hyperbole? And it is hyperbole. Dragonlance didn't ruin everything, but it did exert a baleful influence over the development not just of D&D but also roleplaying in general. In part, that's because it was such a brilliant idea and succeeded so well at its intended goals as an early foray into the creation of a "multimedia" campaign for an RPG. They didn't call it such back in the day, as the term hadn't been invented so far as I know, but that's what it was.
Dragonlance wasn't just a collection of adventure modules; it was also a series of fantasy novels -- phenomenally successful ones at that. One of the things people can easily forget is that, before D&D, and especially before Dragonlance, the "fantasy" genre was pretty small and paled in importance compared to science fiction. Sure, there as Tolkien and Howard and handfuls of pulp fantasies left over from the late 60s and early 70s, but, by and large, most "fantasy" novels were written by SF authors and they weren't (generally) what people would recognize today as fantasy novels, either in content or style.
Dragonlance changed all that. No, it didn't do that alone, so please don't harangue me with dissertations about how the Shanarra series came out seven years earlier and sold X number of copies or whatever. I am aware of these things. Remember, this entry is partially hyperbolic. But here's the thing: whereas Shanarra and Dragonlance are both quite obviously Tolkien knock-offs in broad outline -- being epic fantasies whose settings are pastiches of Middle Earth -- Dragonlance, by being associated with D&D, is the one that probably formed the imaginations of more future fantasy writers. This next generation of writers would, instead of imitating Tolkien, imitate Weis and Hickman, thereby starting the process by which D&D -- and fantasy RPGs in general -- would be snakes swallowing their own tails creatively. That process continues to this day, with D&D ever more influenced by its creative progeny rather than either cleaving to older traditions or creating its own.
On the gaming front, though, Dragonlance commits even greater crimes. Firstly, the original Dragonlance modules were unique in that they presented not merely an entire campaign story arc -- what we'd today probably call an Adventure Path -- but, more importantly, it was an arc written for specific characters who had specific fates. Gamers love to bemoan "railroading," but few adventures were as railroad-y as Dragonlance, a series whose every dramatic element was mapped out in advance. There were, as written, few or no provisions for deviating from the planned story arc or introducing new characters into the saga. Goldmoon would always be the first cleric since the Catalcysm, Raistlin would always slowly descend into evil, and poor Sturm Brightblade was always doomed to die.
Now, at the time, Dragonlance was an amazing innovation. I don't believe there was anything like it and, playing through it, even within its heavy-handed restrictions, simply felt awesome -- especially if you were smart enough to pick one of the cooler characters who didn't die due to dramatic fiat. For the first time, D&D had its own Lord of the Rings, complete with D&D staples like metallic versus chromatic dragons. What wasn't to love? Plus, the Dragonlance modules were well presented, with gorgeous art -- they even made a calendar -- terrific 3-D maps, and a host of hand0-outs and other paraphernalia. In short, Dragonlance was exactly what D&D players had wanted for so long and it was a huge success for TSR.
And like many things D&D players want, it turned out to be a deal with the Devil. From that point on, "story" came to dominate the way D&D and other RPGs were presented. No longer were adventures "modules," implying they could be swapped in and out of campaigns with minimal impact. Now, they had to tell a coherent narrative that was dramatically satisfying. Instead of "just a bunch of stuff that happens," adventures had to make sense. Worse still, the success of the novels meant that, inevitably, there would be more novels and these would alter the game setting in ways meant primarily to sell more books, not necessarily to retain the coherence of the original adventures. Railroad-y the modules may have been, but at least they ended and the players could take pride in having shared in an epic story with a beginning, middle, and end. The novels, though, ensured that the modules meant nothing and that no victory, even one scripted from the start, would remain so. Newer and bigger Cataclysms would be visited on Krynn on an annual basis to sell more books and whatever charm Dragonlance had as a D&D setting was sacrificed on the altar of a profitably exploitable IP.
If any of this sounds familiar, it should. Dragonlance represents the point where D&D definitively took a turn into becoming not merely a brand name with which to sell lots of things unassociated with roleplaying but where TSR decided that the mere making of money was more important than making money by selling fun games. Very few people realized this at the time. All they saw was an epic storyline illustrated by Larry Elmore and Keith Parkinson, with novels whose sales put them regularly on the New York Times bestseller list. I don't think anyone can be blamed for thinking this was all a good thing. D&D had become a huge mainstream success and Dragonlance was a vital part of that. Dragonlance also set the tone for much of what's happened in the hobby since then, mostly for the worse from my perspective, but I'm sure WotC's book department would say otherwise.
Like all "unified field theories," the one I present here has holes large enough to drive Mack trucks through. I don't mean to imply that all that's wrong in the hobby today -- i.e. all I dislike about the hobby -- can be directly connected to Dragonlance. There were precursors to it that set the stage and there were successors that did what it intended better and probably more thoroughly. However, Dragonlance is an important touchstone in the history of RPGs. It certainly represents the end of "old school" as a mainstream part of the hobby. It also marks the time when the fate of RPGs became tied to non-RPG products in a definitive way. I think it's fair to say that, as a hobby, RPGs exist in a post-Dragonlance world, for it changed roleplaying's face forever.