Who shouldn't play D&D?
Bigots would be a big one, but I assume things of that nature are a given, so I'll move on to more interesting ones.
People who need to be the absolute best. I don't mean people who like optimizing (in part because yours truly likes a little optimization himself), I mean people who specifically need whatever it is they're playing to be better than what everyone else is playing. The people who need to feel like they're "winning" even in a cooperative game. They're a plague on both game design and at-the-table play.
People who have no interest in actually interacting with others. D&D isn't a solitaire game. It never will be. You're much better served writing D&D-inspired fanfiction or engaging with jumpchain stuff or the like. Folks who just don't want to actually play with other people--not even a single other person (because "one player and one DM" is a valid approach to D&D, albeit a difficult and often poorly-fitting one)--should not play D&D. It is, inherently, a social activity to at least the absolute bare minimum extent that it requires two people to talk to each other and share their thoughts and ideas.
People who refuse to understand that meaningful conflict is the soul of drama, that meaningful limitations breed creativity, that meaningful flaws create interesting characters, that meaningful challenges leave lasting memories, that meaningful choices are essential to investment, etc., etc. There's an awful lot of advice out there that leaves out that repeated word: meaningful. It is outright false to say that all conflict necessarily leads to good drama; there's an absolute crapload of bad drama built on crappy foundations and uninteresting conflicts (the eight dreaded words: "I don't care what happens to these people.") If all limitations always produce creativity, then we should obviously play with both hands tied behind our backs, blindfolded and gagged, because that clearly creates just about the most maximal (non-permanent) limitations we could have, right?! Except that that's obviously ridiculous. Same goes for all the rest.
This one's gonna be controversial, but I strongly believe it nonetheless. People who make an argument based on the Stormwind fallacy (that optimization prevents roleplay; that roleplay prevents optimization; or that "bad" as in badly-built characters are necessarily richer or better or fuller or more interesting than "good"/well-built characters), the Oberoni fallacy (no game system ever has problems, because if it has a problem, the DM can always Rule Zero it away), Reductio ad videogame (insert game of preference here: WoW, Diablo, whatever, because obviously video games are the worst thing ever?), or DM/player arms race excuses ("it's fine if players can do stupid BS, because that stupid BS can then be used against them by the DM"). All of these arise from arguing in bad faith. There are ways to make every one of these arguments without bad faith: "when players excessively focus on optimization, it pulls them away from thinking of the game as a living, breathing world, and these rules give lots of incentive for optimization" is a perfectly valid argument that doesn't commit the Stormwind fallacy; "you call this a problem when it isn't actually a problem at all, you're wrong to call it so" or "whatever problems you see in this are very clearly rare edge cases, and rare edge cases are one of the most important applications of Rule Zero" are both perfectly valid alternatives to Oberoni. Etc.
And, finally: People who refuse to consider that gameplay styles they personally don't like are part of what D&D is and should be. I find most Gygaxian-style ultra-logistical dungeon-heisting to be painfully, painfully boring, the equivalent of filling out one's taxes in triplicate in order to listen to one's favorite song...each and every time you want to listen to that song. Doesn't mean that style should be given short shrift; it means that the rules for it need to be opt in, rather than opt out. As much as possible, opt-in rules for various playstyles should be supported, hence my advocacy for robust, well-made, accessible "novice level" rules despite the fact that I have negative desire to ever play or use such rules. Folks who think that sort of thing is a pointless waste of time are not, in my not-so-humble opinion, productive voices within the D&D community.
