Okay, let me start by saying that I'm not intending in any way to tell Eberron fans "you should like it the way they did it." I'm not well-versed in Eberron, so my opinion of how well it fits Eberron would be irrelevant. That being said, I'd like to address the issues of what WotC
should do with their product line. (Spoiler: I think they are doing it right.) Also, I only quoted a couple of posts to illustrate the issues I'm responding to, so I'm generally discussing the issues rather than actually responding to any particular posts.
1) I am fairly disappointed with what WotC released in the UA for Eberron. I have seen better home-brewed posted information in EN World than what WotC put together. Why did they even bother?
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b) they have locked down the rules (as another poster suggested) for 5th edition and are terribly reluctant to make any variant changes with regards to the various play settings.
I think that is the key issue here. WotC is keeping a very tight reign on any rules expansions. If they
can represent something "well enough" with an addition of a feat, subclass, or maybe tiny tweak to the existing 5e rules, that's how they
will do it. They are not going to reinvent the wheel for a new setting. Even if they can't represent something with extra feats, subclasses, or tiny rules tweaks, they will make the minimum rules necessary to hit that "well enough" state, or they will not convert it at all and just let fans have fun doing it themselves. We cannot expect 3e or 4e level of options in 5e, or we will be sorely disappointed. It's not that edition.
Personally, I'm a fan of this direction as it keeps to the spirit of how 5e is designed and the "no bloat" product marketing philosophy, which
keeps the game accessible to newer and more casual players--as well as being sensitive to those who want/need official rules but aren't interested in hunting for houserules and doing them themselves.
Why say "Changlings can only change into humanoids that they have previously seen." Really?? What a can of worms THAT is. I can see it now: players are going to be arguing with their DMs about what races they have seen or could have seen in their lifetime. How could they leave that so open-ended and limiting at the same time?
From the MM, it's apparently the same way doppelgangers now work. Whether that is good or bad is a matter of opinion, but at least they are being consistent. It would be odd if changelings were better at a doppelganger's schtick than they themselves are.
Changelings are descended from dopplegangers, therefore obviously have a watered-down version of their main ability. This repetitive complaint going around that you can't change into anything your mind conceives would make a starting character overly powerful beyond the rest of party. It's not practical in 5th edition's theme of play.
I may not be reading your post right, because I'm not sure how they could be overpowered with this feature. Mike Mearls clarified that the change is purely cosmetic--you don't gain any abilities from the assumed form.