I played a lot of Planescape back in the day, and one of my earliest 3rd Edition campaigns was a plane-hopping all-drow game (by demand I swear), but I do not think I really started enjoying the planes until 4th Edition. Partially it was because its cosmology had more of a mythological resonance than the Great Wheel, partially because it was more conducive to actual adventuring. Of course given that it is 4th Edition I am not surprised that it is the only cosmology that does not get mentioned.
For the most part I think it is at best pointless for the core rules—or any setting neutral book—to reference other settings in any capacity. Maybe as a sales pitch in the Dungeon Master's Guide for the busy/lazy/new Dungeon Master, or if a book with new races comes out after the setting book and you want to give players and Dungeon Masters ideas on how to include them. Otherwise I do not want or need a wall of text telling me how halflings are like this, except in this or that setting. Just give me the default assumption—which I will use or ignore as I see fit—and if they differ in another setting make it clear in whatever book is actually appropriate.
Quick aside: I know that warforged debuted in Eberron, but they were included in setting-neutral monster books in 3rd Edition, and were featured in 4th Edition's Monster Manual (which also included a PC race writeup in the back). So, I think at this point it is safe to divorce them from the setting; even 13th Age and Dungeon World have a warforged race (even if by another name for maybe legal reasons).
Similarly I think it is a really, really bad idea to make monster flavor "all-inclusive". Why? Just pick a default story, maybe go with 4th Edition's approach of several unconfirmed origins, and if it is appropriate for it to differ in a specific setting just put it in there. You would think that this would not be difficult, but then past Wandering Monsters articles indicate otherwise.
CONNECTING THE WORLDS
The idea of connecting all the campaign settings always seemed equal parts silly and cheap marketing gimmick, kind of like one of those old cartoon-crossovers that no one remembers: "You can use Dark Sun content in Ravenloft, just make up some bullshit way that the characters wriggle through dimensions!"
I never got into Spelljammer, but I did run a 4th Edition campaign that was pretty similar in concept. I guess it could be cool to see an updated setting since it has the potential to be very different from other settings, but I would at least prefer to not need other setting books to get the most out of it. If you did not before, then awesome! Otherwise meh, been there, done that.
I think what really bothers me about this section is that he not only explains this as if it is necessary to make sure there are ways to connect all the campaign settings (and assumes that they should be), but also acts like these are the only ways that they could be connected. 3rd Edition's "truly expansive view of the multiverse" allowed you to create entirely new cosmologies? Nothing was stopping you except for your own entirely arbitrary, strangely self-imposed limitations.
I do not think that all of the campaign settings should be assumed by default to be connected (seriously, why?). I do not even think that there should be a default cosmology. If you are so keen on doing what older editions did, then why not do what 3rd Edition did and just give us the tools to make our own cosmology? You are already pulling from 3rd Ediiton's Manual of the Planes, why not go the extra mile and include some actual customization? Oh...right.
For me one of Dungeons & Dragons' appeals is not only that you get to build your own setting, but that it was kind of assumed you would. This new direction of apparently making sure that Forgotten Realms at the least gets name-dropped in everything, as well as assuming that everything is connected through some arbitrary planar logic rubs me wrong. I get that I can easily ignore it, but I also do not get why they are doing it.
RACES AND SUBRACES
The entire opening paragraph is hilariously stupid. The first sentence tries to claim that Planescape and Spelljammer were appealing because you could create any character you want, from any setting, whether it was...wait for it...published or not.
Because, what, I could not do that in any other setting? Oh, wait, I could. I mean all it takes is a level-headed DM and a somewhat rational, thematic concept and you should be good to go. This actually makes me think of how 4th Edition's detractors complained about it's message of trying to work with your players somehow prevented DMs from leveraging any kind of authority. Like, now players were free to pitch any absurd, inappropriate concept and there was nothing you could do!
Except, well, tell them that it was inappropriate for whatever reason. Even Dark Sun "dared" to mention that gnomes and divine classes were by default not allowed.
The second end caps the list of creatures with "theoretically warforged". Kender and shield dwarves are obviously in, but we had better make sure that the traditionalists are aware that the warforged are only theoretically allowed because they may not take kindly to such new-fangled things. I do not recall warforged boobs, so maybe they will give it a pass? Honestly I really want to know why warforged are only theoretically okay, but apparently the more interesting question that no one asked is, "what's a shield dwarf?"
The short answer is a dwarf subrace from Forgotten Realms. The long answer is the same thing, but it takes just over a paragraph for him to get to. The actual point he is trying to make is that they think it is so very important for them to make references to other campaign settings (namely Forgotten Realms) in setting neutral books for no discernible reason other than to make fans aware that Forgotten Realms is going to get a ton of support, and they fixed it, and the game is mostly 3rd Edition so pretty please come back?
Seriously, they want to describe a subrace and then describe it in another setting, with another name, but then clarify that both subraces are the same thing.
As with all the other setting specific content, why not just keep it in that setting? Why would I need to know what dwarves are called in Forgotten Realms unless I am, you know, playing in the Forgotten Realms? I do not think it is important to stress that gold dwarves are actually hill dwarves, especially outside the context of that campaign setting. It just sounds like a confusing waste of space.
Really the only part of this article I kind of agree with is differentiating races on a cultural level rather than a mechanical one. Eberron did a fine job of depicting races with the same features and different cultures. I think it is a much better approach than 3rd Edition's Forgotten Realms and its 20+ elf subraces. Of course differentiating them through mechanics is not always a bad thing; I recall races in Dark Sun having differences that were better represented with variant mechanics.