Quoting a little out of order so hopefully things are a little clearer.
I like the depiction of golems and mentioning the controlling spirit being elemental, but I wish they'd left off it had to be from Elemental Earth. We could get some interesting variants if different spirits could be the controlling force behind a golem. Perhaps Earth's big thing could have been spell resistance/immunity. Air-powered golems might be swift. A stone golem with a fire spirit might have magma for blood... and so on.
Bolded for importance (and all me

) But that is basically what I was saying. I would have thought that due to this feeling of inclusion that they would have left it blank, instead of assuming that elemental earth was a thing in all games. Not mentioning is not the same as a new fiction of the same old creature. Saying succubi are still fiends and not specifying devil or demon is the way to make 4e and non-4e people happy. But I digress.
Well, I wouldn't bet $100 on it, but I'm pretty sure the elemental spirit thingie has received at least passing mention in every edition from 1e on up.
Yeah, I just read my 3e MM and was surprised to see that line in there. Guess it slipped my memory of the creatures. Just struck me as odd that out of the many many details they could have included about golems that they figure "from elemental earth" was important.
Regardless, though, that is hardly the point- that doesn't address your question. I'd say that, for many campaigns, it doesn't add a thing.
The "what does it add" mostly related to the above. If it is a line that only works for the old school people then it is probably lost on the new school people. Why include it or specify it either way. You think if they neglected to add the line (or made only the briefest of mentions when the book came out) that old school people would rebel? I think in this way non-comment is better than outright advocacy. But it isn't major enough either way.
If it doesn't improve things- and honestly, I don't see how it would- all taking a little flavor out does is genericize the monster and make it harder for that group where the elemental spirits matter to convert to 5e (or whatever version of the game).
This is getting a little metaphysical on me. If it has some barring or relevance then by all means it should be kept in (or added). If not then I don't get the point. Would the description of the creature be aided by calling them "robots" or defining them in a modern sense? Maybe, but that would be best only for a small selection of the population who don't mind that level of immersion breaking. If it only works for a small group then I have to question the overall value.
2e, until fairly late in its lifespan, disregarded a lot of D&D history
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Even the words "demon" and "devil" started to sneak back in, at least here and there and in some ads (remember "What the hell is a baatezu??"). Likewise, 4e ignored a lot of D&D history- suddenly succubi were devils, for instance- in favor of a more self-contained and consistent world.
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and a significant rewrite of the planes is only going to aggravate Planescape players and make them turn up their noses).
Consistency is important. Why is why I don't advocate for outright changing that core when they are trying to recreate a feeling of old editions. I just wonder if certain things should be specified. Look at my succubi = fiend, not specified devil OR demon kind of idea.
On a broader point: This is mostly true if something has had an inconstancy past, it is hard to say it is ALWAYS canon or always the same if even 2e didn't think it was. Excluding 4e or even 3e I get, but how many years can something be contested and always considered exactly one way, or exactly the way that something 'should' universally 'be'?
I like the great wheel. They have put a lot of effort into it. I love the complexity and detail. I also think that trying too hard to appease planescape people will probable meet with ruin. Planescape people seem happiest when things are a little half-crazy. Beyond that, there is a difference between supporting a setting and forcing a setting's specific details on ALL settings. Why not embrace eberron or dark sun's specifics and override the planescape stuff?
TL;DR version- If it's tradition, changing it WILL alienate some existing gamers, especially if that element (the elemental spirit animating golems) has been important in their campaign. There ought to be a far better reason than "Well, it doesn't add anything to my game" to make a flavor change like that- especially as the flavor does no harm.
How often is that detail important? I hadn't realized that was the reason why golems went berserk. See, I learn things new all the time. If elemental spirit = reason for them to go berserk then it DOES add something to the game. Granted it is something I've never really enjoyed and I'll continue to ignore. But understanding why something is important is the best reason to ignore it, instead of just blindly assuming it is important but not understanding it and banning it. You know, like 4e did with cosmology.
The magic immunity, though, I'm not so sure about. Seems like a really bizarre thing to have universal to all golems, tradition aside. Is it actually based on some existing fiction, or was it just a feeble attempt to reign in casters in editions where they otherwise reigned supreme? Because otherwise, they just seem like magic robots, and I don't get why that would make them immune to magic.
Actually, I know that DnD history will contradict me on this, but I've seen golems (or greater constructs in general) to be more along the lines of animated suits of armor. Less "robots in disguise" or robots in fantasy setting. Warforged always struck me as the robots or rather androids of the construct clan.
Also, am I imagining that there used to be clockwork constructs? Because those would be cool to see re-emerge.
As long as magic and martial are generally balanced (and Next is a failure if they aren't), then there's no reason to have "screw casters" monsters, and I think the blanket immunity should go away just as the critical hit immunity goes away. Have specific immunities/vulnerabilities, sure, but I really don't see why an Iron Golem should be immune to Disintegrate, for example.
A. How will they be balanced?
B. Why shouldn't there be "screw casters" creatures? Aren't rust monsters a "screw melee" creature?
C. The descriptions of those spell immunities go back at least to 3e and I image much earlier. They seem like a fairly old school invention.
D. If I read correctly, then the golems are also kind of a "screw melee/martial" creature too.
E. Kill their controller, avoid the golems IS a winning strategy. They can't be reasoned with because they are preprogrammed, so avoidance is the best way of dealing with them.
Personally, I think 4e's biggest strength was not being afraid to create new, or rearrange old, fluff. Having played since 1977, it was quite refreshing to get something different and thinly framed out to detail or not how I chose to do so, instead of getting yet another regurgitation of "canon". I found it very much in the spirit of OD&D games of my youth when these were just ideas and a few proper names to give some depth, or ignore totally at the DM's whim,.and not the official backstory for all who play D&D to "abide by" since Ad&D was released.
I think the problem I had with 4e is that they did it half way which is annoying. They partially followed the canon instead of throwing it ALL out and starting new.
5 alignments which kept LG and CE, but tossed the rest just frustrated people who understood what LE and CG were, and even the NG and NE people, not to mention the LNs and CNs who didn't think they belonged to unaligned. If you are going to do it, do it the full way.
It is like DC's new 52 - god still hate that reboot - if you want to make a new universe, even to the exclusion of the old, then do it fully. Get rid of or completely revamp your entire cast. Don't say everything in canon is still canon, unless contradicted. That just frustrates the new people who are confused by old references as well as old people who get those references and how the new stuff is doing it wrong. Barbara Gordon was still shot but she got better? What about all that time as Oracle. Why is there a bat-internship? What happened with this crisis or that event. Again, some of it happened some of it didn't. Now, instead of operating on a new premise you are just unsure about what the foundation is.
How this applies to 4e: Succubi are devils now, not demons. But devils and demons are specifically referenced, and they hate eachother on a philosophical level. Archons are now elementally (which I'm sorry they should NOT be), but you still call them Archons. Tieflings are now a race that came about on a different scale and method than people are used to. Eladrin are now High(or Grey depending on setting) Elves. Things like that. Do it fully or not at all. Don't just take some pieces then assume people will be okay with the new version, because newer must be better.
This is even the problem they are especially having now, trying to reconcile the 4e stuff with older material. Saying "which is the best version of this name" and which should go forward. Usually in those kinds of polls KEEP BOTH (new and old) is the correct answer, but there has to be a new definition then. If they had come up with that new definition at the start of 4e instead of reusing old terms they would have made miles of progress to nip those kinds of objections in the bud before they started.
If you are going to paint a room, don't just take a new colour and patch up the holes. Paint the whole goddam wall! Probably several walls (all within the same room) makes the most sense, but painting a single wall is at least necessary. That is NOT what they did in 4e. They just made minor tweaks to canon and then assumed people would silently accept the new version without question. They didn't and now it is a problem they don't know how to resolve. This applies to classes, ranger =/= archer to the exclusion of fighters as archers. There are too many "painting holes but not walls" examples to give.