Here:
First of all bear in mind that the book reflects the state of the art as of Aug 2007, before the announcement. It still has May as the release date. It is full of fluff and art. Sometime a whole page is just a picture, so don’t expect longish text. There is no mechanic, just a few mentions that hint at mechanics.
Ability scores
Ability score adjustment have a net positive benefit (i.e. not of the type -2 / +2). I think that it will be a flat +2 most of the time. In the entry for dwarves it says that the charisma penalty is gone, thus they might have +2 Con, and that is all.
Races
Each race seems to have a clear “homeland”. Dwarves live in mountains, elves in forests. Now they come up with something for all major races. Humans are tied to plains, Halfling to rivers (swamps and marshes), Dragonborn to deserts (at least their great empire were in desert).
I will list a favoured class. As we know, mechanically it is not the favoured class of 3.x, but they admit, that some races are better at being certain classes. The “favoured class” is not listed as such, I inferred it from the text.
Human
They will remain the flexible, adaptable race as in 3.x.
Their negative personality flaw is corruptible (I don’t think it is mechanic, just fluff or the basis of some racial feat).
The text mention that humans never give up, and try thing again-and-again; this can be a racial characteristic (allow retry in some cases).
Each of them know how to handle at least one weapon (fluff or ability?).
Their homeland is the plains. Horses are important to them.
Favoured class: They fit to all classes equally well (or at least more or less equally well, the text is not that clear hear).
Dwarf
Resilient industrious folks.
One of their racial feat allow a second “second wind”.
They no longer have darkvision, only low light vision (most races will have only normal vision).
Favoured class: Fighter
Eladrin
They are the magical high elves. Both elves and eladrins were elves, but one preferred magic and stone, while the other nature and the woods.
One of their racial feats allows them to briefly enter Feywild and reemerge at another place, making a short teleport (was hinted at one of the playtest report, right?).
Favoured class: Wizard
Elf
Favoured class: Ranger
Dragonborn
Dragonborn are kin to dragon, they are egg laying (so why on Earth do females need a halter?). They are a strong race with martial incline. I think dragonborn replaces half-orc as the main damage dealing race.
At higher level they can choose racial feat that gives them breath weapon or wings.
Probably have claw attack.
Favoured class: warlord
Halfling
Now they admit, that the main source of inspiration for the 3.x halfling is the kender. We suspected it all along. They are still the lucky, bit thievery race as they were. Added their liking for rivers and boats (and thus being a merchant and wandered).
Favoured class: Rogue
Thiefling
Lots of fluff, but really not so much crunch. I never played them, so I don’t know how they were in 3.x. Their look is rather unified now.
Favoured class: Warlock