Oh, wow. GW kinda went off the deep end with this one, I think.
Still working through this.
This army has its own unique Force Org chart. Get used to explaining it to your opponent.
You can take an Inquisitorial detachment in one of three ways, primary (elites become scoring in this case), secondary, or tertiary.
Tertiary means you can take them in addition to everything else. So yes, SM + IG + =I= is legit. They have their own Allies matrix.
You can choose to use your Inquisitorial HQ as your Warlord instead of your primary detachment.
One trait table for each of the big three Ordo. Of note is the lack of Fear (which is good, as Fear is 90% useless normally). All three have an Orbital Bombardment one-shot as a Trait.
Trait #s 1, 2, 3 are identical across all three branches. They're all generally useful, although #2 is slightly less useful if you and your opponent don't play with mysterious terrain (why you wouldn't is beyond me, it's part of the core game rules).
4+ is where they start diverging. All 3 get some sort of Preferred Enemy. Xenos is the most generally applicable.
5 - gives some related buff. This is a wasted slot for Malleus if your oppoent fields no Daemons and Hereticus if you opponent fields no Psykers.
6 - Also okay. Malleus gets a useful buff if he's a Psyker and something occasionally useful if he's not. Xenos gets something that's useful, but you probably already had sources of it in the army anyway.
All in all, I'd say they're roughly equal, but Xenos probably gives the widest range of useful entries on the table.
The Liber Heresius is somewhat useful, giving special rules (each rule once per game).
Grimoire of True Names
A nerfbat to Daemons, but only if the character is in a challenge.
Tome of Vethric
Gives a special rule based on what codexes your opponent are playing with. One rule given for each detachment. Useless if your opponent is playing with only human factions or Daemons.
There's really only 2 choices for units: Henchmen Warbands and Inquisitors.
Coteaz is here, looks like a copy/paste from C:GK. Has a fixed Warlord trait. Same with Karamazov. He's still an auto-include for competitive play for all the usual reason, although not in a Primary detachment.
Karamazov's Warlord trait is kinda meh, but he gets one of the other ones as a special rule, with a bonus.
All three Inquisitors can take some specialty wargear, looks mostly C/P'd from GK. They can take static psychic powers or roll on one of three tables, same as in the GK FAQ. Hammerhand (known locally as the pimp's prayer) is still around. They all start with carapace and can buy up from there (Malleus gets Terminator armor, ho ho ho).
These are your only other choice in the 'dex. Good that you can do pretty much anything with them.
Acolytes are your general-purpose combat models. They're cheap and somewhat upgradable. Since they can take power armor, feel free to use those Sisters models! IG Stormtrooper models would also be legal options for the most part.
Inquisition got the Adepta Sororitas Priests as part of their warband. Pretty awesome.
Arco-flags/Crusaders/DCAs are almost certainly a copy/paste from GK (points and all), which irritates me. Adepta Sororitas Ecclesiarchial warbands have cheaper Arco-flags but explicitly power swords, while it's back to generic "power weapons" for Inquisition. Argh.
Mystics/Psykers/Jokaeros are here too. Fixed power on the Psykers.
Oh, wow. Valkyries, Razorbacks, Chimeras, Rhinos, and all of the Land Raiders! All of the options you'd expect are here, with the option of being able to take mind bullets for the Chimera. You can't take stubbers on the Chimera, though. Psyflame options are available for the Razorback and Land Raider Redeemer.
Truesilver armor is lol if your oppoent brought CC Psykers/Daemons.
All in all, looks like a rip from GK with extra fluff. Being able to take Valkyries is hilarious, along with Psybolt Chimeras.