The College of Shadows bard and the Circle of Dust druid are problematic. I look back on them now, with the bushy-eyebrowed frowny-face of an overly critical creative person, and mutter garbled curses in the forgotten tongues of the elder races. I could beat myself up over it, but really I think the crux of my problem is not having enough official products to establish a pattern in their design yet. The UA articles give me an idea, but they shouldn't considered final products by their own admission.
Off the blog, I've been working on updating an old homebrew setting of mine (because you're not a Dungeon Master if you didn't have some stupid malarkey you thought was a good idea back when you were fifteen and now you want to Do It Right), and creating character races from that setting is actually much harder than I expected it to be. What's balanced? What's fun? What's the Right Way to do it from a rules perspective?
To that end, I bought a couple products recently -- Midgard Heroes and Southlands Heroes, both from Kobold Press. They're not first party books, but the designers put a lot of effort into them and while the settings from which they hail are not something I'm likely to play (despite their coolness), their races are fun. Some are extrapolations of monster races into player ones (i.e. Centaur, Gnoll, Kobold, Lizardfolk), or totally reminiscent of things they might not have been intended to be (tell me the Alseids aren't directly translatable into Bariaur from Planescape and I'll call you a dirty liar).
Some are crazy new and have some awesome design ideas, like the Darakhul Ghoul whose traits are partially contingent on what player race they were before being turned into ghouls.
There are a couple tricks they used that I really like. For example, the Tosculi waspfolk in Southlands Heroes
Their Lizardfolk from the same book have the same idea, but the alternate racial traits are there to emulate Lizardfolk who come from different tribes inspired by other reptiles, like geckos, chameleons, et cetera.
I think this is a fun design space to work in, and it helps me quite a bit in determining how I'd build some of my old race concepts in the new edition. However, I'm a bit sad that the idea of Racial Feats doesn't seem to have taken hold, because I think there's equal usefulness there that hasn't yet been tapped.
I suppose there's evidence to suggest that they don't want to paint all Dungeon Masters into a corner so they must use feats (where some might not want to; it does list them as optional in the preamble to chapter six in the Player's Handbook, after all). Still, they naturally use a gating mechanism (player level advancement, unless you wanted to have variant humans and human racial feats for your setting) and a rules plug-in that's already comfortable and established in the minds of its veteran players.
Anyway, the two books bring up some ways to handle future conversion stuff on top of all that. Their Centaurs are Large creatures, which immediately sends up flares about combat balance; between reach and fighting space, this can create severe headaches. However, they do logically call out that the "human" part of the Centaur is still no bigger than you'd expect and they still only use normally-sized weapons and armor (and have no reach).
You may be saying, "Well duh, Marsupialmancer," but this honestly didn't occur to me until I saw it here, which is part of the problem. I'm having trouble determining what consistencies are safe to mess with, in this case Large being Large was something that was safe and logical.
I still don't know what Wizards considers safe yet, so I am taking my cues from those whose day job this is at this point. I'm still not entirely comfortable messing with what I think are the established boundaries yet.