Racial mechanics in D&D have really always been a bit wonky from the very first edition, so this is nothing new, and it is admittedly difficult to come up with abilities for near-human species that give them an edge. What exact effect does a human who lives underground, is a foot shorter than normal yet is bulky enough to weight 1.5x as much have on the mechanics of the character? What about living for over a hundred yets and yet still having the body of a fit 15-year old with pointy ears? What about being a race of humans universally afflicted with dwarfism?
I just really think that if some more time is put into it, that something better can be come up with than what we see in the playtest packets. Yes, these abilities have traction in previous editions, but that doesn't mean something new can't be put in its place.
Immune to Poison - Immunity is a pretty absolutist thing to put there. Immunity means that there is no limit to the amount of toxin that Dwarfs can withstand, that there is no way to deliver it that would affect them. With this ability, a dwarf can take a bath in Scorpion venom, they can walk right through a cloud of poison gas with no ill effects, they can drink giant viper venom as a delicacy, make it a typical ingredient in their food.
With this ability, when playing a dwarf I would really make use of poison as much as possible, coat all my weapons in it even if it isn't a feature of my class and I have no skill in doing so because even if I miss my roll and injure and are supposed to have "poisoned" myself, I am immune. So I'll just do it again and again until I succeed. As a DM, if I am building a dwarven stronghold or city that isn't meant to be open to outsides, I would go ahead and have them pump every room full of poisoned gas and put in moats of toxin water. There'd be no better defense against their enemies.
Slower Speed + No Armor Speed Reduction - You know, you can really go ahead and reduce these two rules to a single rule -- Dwarfs not wearing heavy armor get a penalty to their speed. Such a rule would have the identical game effect. I understand that the basic concept of Dwarfs is such that are always wearing heavy armor, but this really is a way to scare dwarfs away from playing a class other than Fighter and Cleric.
I can understand a designer giving a penalty to a race that only affects a couple classes, but only in a case where their attribute bonuses match the class so well that they would become the best racial option for that class, particularly when thematically they aren't meant to be particularly good at it. But in this case the rules are penalizing a Dwarf even further only in the case that they play classes where their attribute bonuses don't really give them any help.
This points to a return to a system where there is a "correct" race to play each class as.
Stonecutting - Okay, not so bad here. But really, what this points to is familiarity with terrain. One can assume that underground caverns are an unusual enough terrain from where most of the PC races live, but a place that is encountered often enough in games that it might be worth noting that they have certain advantages for figuring out their surroundings. But this really points more to a background trait than a racial one. I would expect that "Mountain" Gnomes and "Mountain" Orcs and "Mountain" Goblins and Drow would all demonstrate similar abilities. Furthermore, I would expect the "Forest/Wood" version of all these races to have similar advantages in the forest and that aquatic races would have similar advantages underwater.
But, somehow I suspect that we aren't going to see the concept of "familiarity with unusual and typically confusing terrain because it is your native homeland" to be copied over onto every racial entity it applied to. Because, if it was, then we would expect to see some tangible advantage to someone who came from an urban environment or open plans.
Halflings are really are a weak concept to begin with. They really have never been anything more clearly defined than simply "short humans". Yes, there have been some attempts to try to really nail them down to something a bit more clear than that, but such attempts have never really stuck particularly well. In fact, it seems like every other small race has a better theme -- gnomes univerally learn illusion magic, goblins like ambush tactics and outnumbering enemies, kobolds like traps and have some reptillian advantages... at this point I'd have to question if it would not be best to simply drop the concept of Halfling and instead adopt a PC version of one of the others.
Because no one really can say what a Halfling really is beyond simply a short human, I'll have to let the whole "lucky" mechanic go. If that is what it takes to try to make them "special" or "unique" then I can't really argue with it-- it is simply too intangible of an advantage to really pin down how it could be wrong. But the other one...
Naturally Stealthy - This seems pointless. The reason is quite simple-- no matter how you look at it, this sort of ability should be universal. What exactly would make a Halfling better at hiding behind something larger than it than any other small race? In fact, what exactly would make a Halfling better at hiding behind something larger than it than any normal sized race?
Previous to having seen this I would have thought that a human rogue told me that he would like to hide behind the Ogre that, for whatever reason, isn't trying to squash him than I well would have assumed it was a perfectly valid thing to do. In fact, I probably would have let any generally human-sized character hide behind even a bugbear or dragonborn or warforged if they so wished to do so (with some more difficulty than using the Ogre or something larger, obviously).
But having it codified as a Halfling racial ability means that... well, apparently no one else is capable of hiding behind a creature larger than it? Because if they could... it wouldn't be written as a racial ability for the Halfling.
Honestly, both of the abilities of the Halfling tell me that the designers are really racking their brain and stretching to try to find something unique there. Maybe there isn't anything. Halflings are merely small-sized humans and so why not simply make them small-sized and then give them whatever mechanic has been assigned to the human? Maybe it just comes down to giving the race a free bonus skill or something. But, again, the trouble with the Halfling is... what exactly do they do better than a Gnome, Goblin or Kobold (other than get along with humans)?
Free Spirit - Assuming that charm and sleep are relatively similar to previous editions, what this ability really translates to is... immunity to low-level autokill spells. Perhaps the problem is merely that these spells are incredibly, incredibly powerful at low levels. Basically anything 1st or 2nd level is just automatically killed using these spells and the sleep spell allows you to auto-kill an entire field of enemies in a 20' radius. Elves are immune yet... somehow I just don't think Elves are going to be common enough first or second level opponents for that to matter. Second-of-all, only a sadistic DM is going to be using sleep or charm spells on the party at first or second level when chances are that they won't get get a save from it and having an elf in the party would be an absolute necessity (in fact, if the DM begins liberally using enemies with a sleep spell at their disposal, the entire playgroup might begin playing elves to survive).
Honestly, I am not even sure if this is broken or not, but fundamentally it is immunity to a low-level autokill spell that DMs should be discouraged from using as written anyway and probably won't come into effect much past 3rd level.
Keen Senses - It is a really powerful bonus when coupled with classes that are meant to be scouting I guess. It just strikes me as a bit odd that with all the really beastial, animalistic races in the D&D world who spend most of their lives as predators or prey and live in societies where not keeping a careful eye out at all times can mean a dagger in the back, it is elves-- the most civilized, altruistic, protected and peaceful ones that have the best senses. Simply having Low Light Vision has already gotten across the idea that they have sharp senses, this extra bonus seems surperfluous.
What really strikes me about how the elves have always been written up is that the most striking feature of the race is mechanically ignored. Hhow exactly does this race live for more than a century, do and learn NOTHING more than most races do in less than 20 years and then suddenly join an adventuring group and see their abilities massively increase in only a couple years?
Previous editions have always given Elves wacky mechanics-- they don't need to sleep, they have exclusive access to the best metal, the best horses, the best hawks, the best hunting hounds, they are friends of the fey, they know their way around underground, they have better hearing than the races with giant ears and better sight than hawks, they get free access to a few cantrips, they are immune to certain spells, there is a special subrace to specialize in every single attribute so they can match every class....
All of this has always been designed to try to convey that they are simply superior to humans (and every other race) in every single meaningful way. But, really, shouldn't the most fundamentally important aspect of the elf be... it has something between 2-5x the lifespan, 2-5x the memories, 2-5x the experience of every single other member of the party?
One can propose the elves as a perfect master race all one likes, to me that additional life experience in which to pick up various skills and knowledge would really be the advantage that should be highlighted.
With that in mind, instead of the Keen Senses, why not instead let an Elf choose 2 backgrounds to represent their sigificantly longer and perhaps more complex life history?