Some more details:
The Temple of Elemental Evil is mentioned as a Location of Note. This section is said to contain 'future adventuring locales'...
Dragons:
Bunches of stuff here, the highlights include:
The different colors of Dragon have different monster roles - in other words, some are artillery (blue), some are brutes (white), some are soldiers (red) and so on
Dragons are solo monsters who 'get to do more on their turn than most monsters do' and also 'get to do a lot when it's not their turn' (like the tail slap we already knew about, but also mentioned is the green dragon's ability to poison you if you get too close)
Dragons have fewer abilities, focused on the most iconic ones (continuing the theme). For example, the oldest black dragon is said to have only five possible standard actions, with unique magical abilities taking the place of spells simply taken from the wizard's lists.
Dragons aren't forced into specific 'alignments'. Their motives can vary from a baseline - chromatic are wild, metallic like to be in control but good and evil manifests in each.
There are two new flavours of metallic dragon which displace bronze and brass from the core group. The new metallics are Iron and Adamantine.
Chromatic dragons grow in raw elemental power as they age which manifests as unique new powers related to the appropriate element. An ancient red's breath weapon, for example, can 'scour the fire resistance right off you'
An all new look and set of powers for the green dragon; they are back to breathing poison!
Giants:
There are Huge versions of the standard giants called Titans - these are more closely tied to the elements and have greater power
Giants in general are more elemental in nature, and there is greater variety between the standard types
The Giant type is specifically for Giants. Trolls, ogres, ettins don't 'necessarily' have the Giant 'type'
Underdark:
Now considered easier to get to
Mentioned are Drow, Troglodytes, Mind Flayers, Kuo-Toas (aboleth servant/worshippers), aboleths, myconids
Vault of the Drow is mentioned as an Underdark Location of Note
Feywild:
Mentioned inhabitants include hags, yeth hounds, centaurs, eladrins, treants, fomorians, unicorns, elves, firbolgs, the Wild Hunt, red caps, quicklings, will-o'wisps, dark ones, pixies
There is a fey reflection of the underdark, ruled over by fomorians
The Isle of Dread is mentioned as a Location of Note
Gnomes are a possible fey-dwelling race
Pixies, well, don't want to ruin the surprise
Shadowfell:
Merges Negative Energy Plane and Plane of Shadow, removing the irritating bits that make these places a pain to visit
Shadow is a power source. Involved with stealth, illusion, dread, 'devastating enemies' and 'necrotic energy'
They've re-concepted the undead, adding the animus, providing 'vitality and mobility', as a companion to the soul and the body.
Very interesting descriptions of how different varieties of undead are now explained. Shadow are the animus freed of body and soul, for example.
Lots of details about how the new cosmology explains resurrection and reincarnation
Shadowfell is inhabited by ... the Shadar-kai.