At A Glance: Pathfinder Unchained.

So, I sez.. 'betcha you won't pull that there chain...'

Pathfinder Unchained is what you get when you let the designers of the Pathfinder game brainstorm things out. Variant rules. Lots of them. New takes on the classes that have GMs tearing out their hair. Options and different takes on a myriad of things: Fractional Base Bonuses (saves/BAB and such), staggered advancement (variant version of multiclassing)

Classes: A new look at barbarian, monk, rogue and summoner.
 -Barbarian: Not too bad. More of a steamline and polish than re-balancing. Barbarian rage is now a simple +2 to melee attack/damage, thrown melee attacks and will saves, along with a penalty to AC, and temporary HP that goes first.. so you don't die from HP loss at the end of your rage. Rage powers are pretty much the same, with a few enhancements. You got a new category of rage powers: Stances. You can only use one stance power at a time, so there will be some switching.
-Monk: A tweak and polish to the monk. Full BAB and a tweak to flurry gives the monk teeth while throttling back the flurry mayhem. The big change (I've seen so far) is the ki powers; they are a very broad and selectable. Very wuxia in outlook. Style strike is added as well. One of the monk's flurry blows can be a style strike (with more as he levels) and have all sorts of fun effects.
-Rogue: One of my favorite classes, though I typically suck at it, but let's be honest there are oh-so-many options on how to do a 'trap monkey' now. You got to do a lot to make the rogue relevant when you got slayers, investigators, and all manner of archetypes and traits to get trapfinding in the mix. Most of what we get is polish, not much outright new. The new is finesse training, rogue's edge and debilitating injury. Finesse training is like what swashbuckler's get, an agile fighting style, but staggered. At 1st level, a rogue gets weapon finesse for free, at 3rd they get to add their dex (in stead of strength) to one finesse useable weapon with others being added later. Rogue's edge plays on the rogue's role as a skill monkey, adding on skill unlock powers to the skills they have. (More on that later) and finally debilitating strike which inflicts a a status condition when dealing sneak attack damage. Adding in tweaks and polish to rogue talents, I think they did a good job of pushing the rogue back into relevance.
-Summoner: If there was a class more hated by GMs than the summoner, I can't think of it. The summoner gets some tweaks and some justifiably needed re-balancing. Pretty much stays the same spellwise, with a few tweaks. Yeah, no more 2nd level haste spell kids. The big changes, naturally, are all around the eidolon. Each one now has a planar subtype as well as a body type. Each is aligned and a summoner can pick any 'adjacent' aligned type. Each gets perks and bonuses. Forms are tied to type: resistances, bonus to saving throws, dr and so on. All in all. I'm a hung jury on the summoner till I get to tweak my summoner and compare him to him as an unchained summoner.
-Factional base bonuses  & staggered advancement: Ways to advance/balance multiclass characters out. Meh to me.

Skills & Options: Background skills, consolidated skills, group skills, crafts/professions, skill unlocks and more on variant multiclassing. A variety of ways to streamline skill usage. Grouping skills by type, condesning them even further, and giving you a reason to specialize in a skill with Skill unlocks. You take a feat: Signature Skill, and go from there. Every five points you put in the skill comes an exceptional way you use it. I like that. Particularly for the rogues who now get one free @ 5th level. Lastly the multiclass alternates. (the stuff in classes is more phased advancement), this is more like the stuff in 4E multiclassing. Which I didn't like.
Gameplay: Gameplay elements like alignment, revising action economy, swapping out iterative attacks, tactics, wound levels and revised poison/disease. Tweaks and alternate methods of game play elements.
Magic: the big juice to me. You get rules for alternate spell casting as well as altering magic conditions (limited and wild magic) as well overclocking spells, spell fumbles, and esoteric material components and a few feats to juice things up. Then the last bit of the section is scaling items, and other approaches to magic items.
Monsters: More toys for the race/monster building crowd. Not my cup of tea but seems clear cut and good for making unique to your campaign folio of monster.

The crunch: Lots of it. particularly if you are a home gamer who wants to break with the usual mechanics.  It will be interesting to see what plays out in PFS and how much of this is followed up on in the splat books over time.