My main problem with FR was the meta-plot, particularly since the formative years of the setting was spent under TSR's Comics Code-inspired "Code of Conduct." This problem showed itself primarily in two ways:
1. Plot threads from the core box and sourcebooks get resolved in novels by NPCs, and later sourcebooks assume these things have been taken care of.
1a. Since plot threads keep getting resolved (because Good always wins), new things have to be thrown into the setting every now and then, leading to things like a I-can't-believe-it's-not-Mongols invasion or ancient cities reappearing.
2. Their pantheon gets turned into a gorram soap opera, with new gods rising, old ones dying, portfolios changing hands, changing hands again, gods thought dead return, and so on and so forth. The rise of a new god should be a momentous thing, not "A new god again? Must be Tuesday."
Two of the defining traits of Eberron are, I think, direct reactions to these issues: a frozen timeline, and distant and possibly non-existent gods.
I love the Realms, and have run my campaigns in it since it came out. One of my favorite things is that plot threads move forward in the supplements and the novels. They aren't always "good wins" results - such as the events in Cormyr, and everything went out the window when 4e came along.
It makes the world a living world, and I always encouraged my players to read the novels and the setting materials to expand our shared knowledge of the world.
When the Avatar Crisis occurred, at the time I thought it was an interesting and unique way to explain why the world worked differently than it had before. It explained why the physics of the fantasy world (combat, magic, etc.) was different than before. I still think that it was a unique and somewhat inspired idea, even if a bit clunky at times. The gods dying and rising at that point was OK too, since it
was a big deal at that time. The continuation of that - Ed Greenwood's
Shadows of the Avatars series was much better than the Avatar series itself, and I didn't mind the idea that the lives of the Gods isn't static. On the other hand, it did get to be too much, and even though I like the idea of gods being living beings, I probably wouldn't have the events in the planes impact the world so directly. The churches of dead gods wouldn't disappear overnight, and their prayers would be answered by other gods. Most importantly, what the mortals believe wouldn't be tied to what's actually happening in the planes. The beliefs and teachings of the church would
not match what's going on in the planes.
What I didn't like was the unintended side effects of these lines. First, the power creep. I recall Ed Greenwood (might have been Jeff Grubb) saying that the NPCs in the Gray Box were given levels high enough that they would "always" be beyond the capabilities of the PCs, and they PCs couldn't just go and slay them. Much like Gary Gygax originally intended spells higher than 7th level for NPCs, so they'd be a challenge and have abilities that the PCs didn't. But once they were out there, it became a sort of expectation. The novels tended to lean super heroic rather than just heroic, and now the expectation was that the PCs needed to be higher level. It also meant that the adventures and events they portrayed tended to be more on the epic side.
The original setting was presented as something more grounded in "reality." It was enhanced by Ed Greenwood's original
Ecology of... series in
Dragon magazine. There were in world reasons why dwarves and elves were rare, and incorporated the framework of the races presented in AD&D. And while many place the blame for power creep and such on Ed, I know that he was (is) under some contractual obligations, for example, Elminster must appear in every novel he writes. And I think the direction was set by TSR more than Ed during the 2e years. Just a guess though.
In addition, when it became the default/official setting, then every new idea got dumped into the Forgotten Realms. The Mongol invasion. The New World (Maztica), etc. This was further exacerbated with the shift to publishing supplements for players instead of DMs. Now each supplement has to have new player options. New races, new classes, etc. All of this has the effect of diluting and altering the setting itself. Yes, the Avatar crisis is probably the point where that started. And when I look back on it now, I like it much less simply because it ended up providing a precedent that gave them license to shake up the nature of the world each time a change in the rules came along.
The only reason I'm using the current 5e timeline is because they've done a lot (not enough) to pull back towards the original Realms. I still want to take advantage of it being a living campaign, although I'm not as closely tied to it now, dumping a lot of things that I just don't want in my campaign. As time goes on it will probably be less and less important.
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As for the OP, I think a portion of it is perception. That is, the haters are more vocal than non-haters. Even the folks that love it, like me, are often quite vocal about what we don't like about changes that have been made. The 4e changes are a perfect example - even those that loved the Realms had a lot to say about that.
Second, since it became the default/official setting, when you picked up D&D you also picked up the Realms, or at least a part of it. Not to get into edition wars, but probably the biggest issue I had with 4e is characterized by eladrin. They decided that what D&D was missing was it's own lore, that they needed to create their own. So they did. And they went a step farther and said that the new lore needed to carry across all campaigns. So the planar lore of the Realms? Gone. Gods? Some are still there, many are gone. Elves? Well, sort of, now for whatever reason, despite the lore describing tens of thousands of years of history of the elves in the Realms, they're all mysteriously wrong. Now there are eladrin. Which also meant that the eladrin that already existed weren't there anymore. This was even before the Spellplague (which personally I don't really care about, it could have been interesting) and Returned Abeir, which I despised. Combined with jumping the timeline ahead 100 years which pretty much killed any existing campaigns, but wasn't nearly enough to explain why the Realms was now like
Neverwinter Nights where every race lives everywhere in harmony with each other, despite the fact that many now look ridiculously monstrous and have only been around for 100 years.
Suffice to say, I think the problem became one of stewardship. The feelings of Realms fans about the 4e changes is a large part of what shows up as Realms haters.
Although there are many calling for a 5e campaign setting, the reality is that most of the best stuff that's been published, starting as far back as 2e, came from the 1e supplements, and the Ed Greenwood supplements from 2e. Large portions of the text in the 3e and 5e materials come directly from the 1e materials and Ed's Volo's Guides. The rest of the material is hit or miss, depending on the author and tie-ins.
I disagree with the assessments of other settings. Greyhawk has always had a distinct personality, just as Dragonlance, Ravenloft, and Dark Sun, for example. What makes them interesting is that they are unique and have their own feel. Since they haven't received as much attention from TSR or WotC, they remain undiluted in their nature. Mystara suffers a lot of the same issues the Realms does - as the official setting of the BECMI series, there was a need for more material. And the quality varies quite a bit.
This is where the 5e publishing approach is likely to help immensely. Instead of being in a rush to produce more material, the quality of such material should be higher. Unfortunately, that's not always the case.
Volo's Guide to Monsters is a great product in terms of concept and design. Unfortunately, once again it's placed squarely within Realmslore, but they've totally ignored prior lore on the monsters and races presented. In addition, since the Realms is once again the default setting (despite WotC denying it), they're dumping everything in the realms whether it belongs there or not.
Anyway, I think there are a lot of people (probably the majority) who just don't care. They buy the APs, play them, and move on. The setting itself, or the setting integrity isn't something they care about. And they don't have much, if anything, to say about it at all.
I think the next largest group is probably those that love the Realms, but can't stand some or much of what TSR and WotC have done with it. This is a varied group, with all sorts of cutoff threshold. This is a very vocal group.
Then there are people who just hate it. Not as big a group, and not as vocal as the Realms Love/Hate group, but always willing to pipe in with a one-liner about what they dislike (Drizzt. Elminster. Etc.).
I think the smallest group is the one that loves everything about the Realms. Everything that has been published. I still incorporate probably about 80%+ into the lore of my campaign, even the stuff I don't like. Because stuff happens in the world that we don't like. But that doesn't mean I like it, and I will voice my displeasure (putting me in group 2).