D&D is somewhat unique in that it's not designed around a specific setting or theme, but instead meant to be fairly generic, allowing the DM to use whatever specific type of game they want.
I disagree.
D&D has a very strong theme: D&D high fantasy.
D&D is very much its own genre.
It has become a
very influential genre that has become so prolific that it seems generic when looked at on its own without hindsight..
Secondly, D&D doesn't have a precisely specific setting, but it has a huge implied setting from the spells, monsters, etc, and it's themes are as limited by the rules as a lot of indie TT RPGs.
Yes. D&D has always been rather coy about having an "official" setting.
But the truth is that Forgotten Realms has basically won that popularity contest. (Not a huge shock seeing as how Greyhawk got made the redheaded stepchild, and the FR was made to encompass all of D&D's legacy system conceits early on way back in 1e.)
And even though 5e continues to be on the down-low about it - classes, spells, cosmology, monsters, are all Heavily biased to Playing D&D style high Fantasy in settings just happen to be very similar to
gasp the FR.
...
And because of HP bloat you also always have the case where the PC can walk through heavy suppression fire just fine, only losing a bit of HP which are easy to replace in D&D, and start punching or stabbing the gunner when in reality you would seek cover and running through it is close to suicide. And because we are familiar with modern settings such differences in behaviour is very noticeable.
The level and HP bloat also affect other things. When a PC is level 10, why would he care what some guardsmen says? They can do nothing against him unless they suddenly turn into level 10 guards for no reason. And in a war game you also have the problem that the PCs can only ever meet level appropriate enemies even in a chaotic field battle because enemies of lower level are no threat and (much) higher level enemies guaranteed TPKs.
HP Bloat is the #1 issue that straight d20 conversions to other non-D&D genre's and settings has always felt off for people. Me included.
HP added at every level has a lot of knock on effects to the system. D&D has always had scaling issues at higher levels that they have never been fully able to address in any edition.
The issues with HP Bloat change the way the game is played as a characters HP increases. And this design and PC advancement paradigm did not match up very well with many of the IP that were being converted to the d20 way of doing things.
If you’re doing fantasy adventure, D&D is great. Fantasy adventure is very, very, broad, including everything from Lord of The Rings to Star Wars to David the Gnome and more.
I agree that Fantasy Adventure is very broad.
Which is why I completely disagree that D&D is great for all of it.
The core D&D paradigm of HP at every level does not match the conceits of certain settings and genre's of Fantasy Adventure very well.
5e is open about the fact that they have 4 tiers of play that encompass a huge power shift for PC's that is not present in RPGs that don't have HP bloat.
RPG's without HP bloat have a different play dynamic.
That being said; Lots of players just don't care about some 'different play dynamic'.
If you just fiddle with the D&D classes a bit so you can say "Here you go a ranger class like Aragorn! Wanna play in Middle Earth for a bit?"
They are all over it.
People like playing D&D with a Star Wars veneer, or a Middle Earth veneer.
I don’t think it’s inherently easier to hack than any other system, really. People are just very willing to hack it, because they’re very comfortable with the base rules, and because D&D has had a DIY culture since its inception.
Yup. Familiarity > System.
Call of Cthulhu has it's own little quirks. As a new Keeper, it was sometimes confusing to figure out whether or not a player should use Fast Talk, Persuasion, or Credit Rating for social interactions as the game left it up to me to figure out which was best for the situation. And is there any reason we have the skills of Handgun, Rifle/Shotgun, and Machine Gun are all separate skills? If my character has a History of 65% that's knowledge covers the entire breadth and scope of human civilization but we need to get into the nitty gritty with weapon skills? And the advice for running investigative games wasn't the best though they've improved this quite a bit with 7th edition.
BRP / CoC has legacy issues that continue because that is the way the system does things. Similar to D&D's sacred cows.
Skills in most BRP games like CoC could absolutely use a big trim / redesign to better fit the genre they are altering the system for.
But Chaosium are likely very leery about doing anything that might cause a revolt in the player base.
Functionally there is no reason for most BRP games to not be d20 roll under like pendragon - but that runs into the same issue as trimmed down skill lists - player revolt...
it’s about the inverted character progression. You could bring Sanity into D&D, but you’d still have a game where the characters face challenges and grow stronger from it, rather than a game where characters are worn down to almost nothing by one harrowing experience after another.
Yes, very different play dynamics.
And some just prefer to have the D&D play/progression mode in every game/genre.
Because they like it.
*
* I think that the reasons they like it have a lot to do with system familiarity and comfort level, but that is almost a different tangent.