I know people like the "relevant to adventuring" details more than little bits of fluff like the goblin Jester, but It's always a nice touch that helps the world feel alive.
As for the races, I'm still not fond of how they handled kobolds and orcs (especially kobolds, I'd much rather a trapmaker ability than cowering) as player races (stat decreases instead of abilities that encourage a flavour) but it has dulled a little over time. The other monstrous races range from decent but don't get much (hobgoblin) to decent and get a lot (Yuan-ti). I don't know if any of them get anything that makes them truly "stand-out" though.
My initial thoughts on Aasmiar were mixed, and they're still a little mixed. On the one hand, they are definitely not just "good" tieflings anymore; on the other hand I was planning to write up some variant abilities for them along the lines of the SCAG tieflings and now have to rethink it.
Firbolg flavour is pretty much exchanging the druid class for a druid race (speech of beast and leaf reinforces this), but it represents something a fair bit different from forest gnomes or elves. Neither of them are outright protectors of nature, they just live in forests. Firbolgs do. Gnomes try to hide their homes, Elves don't care (they're just secluded), while firbolgs try to hide they they're firbolgs, but will step in to try and prevent their groves from being harmed. Mechanically their traits represent someone who conceals themselves with the disguise self spell, or keep themselves hidden with an invisibility causing trait. The rest reflects being a nature giant.
Goliath is a reprint of the EEPC. I don't know what it is, but they're missing something to help them escape the shadow of being tall humans and a different way of playing a half-orc. Perhaps something that really drives home their nature of self-dependence beyond what the flavour does. I don't think they're bad as far as playing them (one of my favourite characters was a goliath fighter named Axesmasher), but they could use a more solid footing.
Kenku have a fairly interesting backstory to hook ideas onto, but mechanically might have a hard time having their choice in race stand in the foreground. The only thing they really have is the ability to copy. That copying, however, covers almost everything from document forgeries to replica trinkets to sounds. More likely however is that the difference between a Kenku and a Charlatan of a different race in play is more likely going to come down to the fact that kenku can't talk normally, and have to mimic. This is going to get on the nerves of the rest of the group if you overdo it though, so even that might not always be as important, depending on how many NPCs you talk to.
Lizardfolk are almost the opposite of Kenku to me in that they have a meh background but really flavourful traits. Their backstory is straightforward (live in swamps and just want to be left alone, don't really understand humanoid ideals sometimes) but they're supposed to have an alien mindset compared to other humanoids (ideally have no emotions at all, but to me seems less Vulcan and more pragmatic). Mechanically though, wow. You might have a hard time taking advantage of everything, but if you can, you'll feel a heck of a lot different from the rest of the party. Running into battle naked to chow down on goblins, and then when you're done fighting you can skin them to make shields and javelins! Warning: Not for mixing with lawful good Paladins.
Tabaxi have an interesting (but not for everyone) backstory of being wanderers from (I assume) fantasy Mesoamerica, who delight in looking at and finding weird stuff. Now, I've heard them be compared to Kender, but that's not entirely true. Kender can't understand property. Tabaxi clearly do, but that won't stop them. Mechanically, they're leopards, maybe with a dash of cheetah, with the ability to climb walls easy and bursts of super speed. And they have claws. They don't quite evoke the background flavour on their own, but they suffice for making you feel different from other party members (especially if you stack it with monk abilities)
Triton: flavourfully sea elves, mechanically Aquaman. They go together well enough on their own.
Tabaxi, lizardfolk, and tritons all have a quirk table, which is what's more likely going to be what captures the background flavour. Firbolgs have a "Why are you an outcast?" table (which not all of you would want to roll on; maybe you're not an outcast), and Aasimar have a quick table for making an angellic being to guide them (mostly as a starting point, you can make your own or have a reason for said guide to abandon you).
