I've got the book sitting next to me, what do you want to know?
In terms of power scale, it's above Dark Heresy but below all the other titles. Players cooperatively build a regiment and then pick guard classes from within that regiment, and go die for the Emperor. Includes rules for piloting and destroying tanks and other vehicles.
Characters start out with more skills and better equipped than they do in Dark Heresy by far. Their weapons are also slightly more effective than their DH counter parts. Being the last thing published, its rules are about the tightest one can reasonably expect from the series (which still isn't saying a ton, but it's way smoother than Dark Heresy.)
Other than that, I think the real difference is the kind of games you end up running. Combat is notoriously slow in all these games, because it's tries to incorporate as many table top war gaming elements as it can. And Only War invites the biggest, most complicated combats of any of the games, to me. The Imperial Guard are soldiers so they're in battles. And unless you're running a campaign based 100% on stealth infiltrator squads working behind the lines, you're bound to end up running a large combat at least once. And by large I mean, several hordes worth. Where a "scene" in the other games might be a city or a starship, in Only War they'd be battle fronts. (Of course you could run any kind of adventure, but these are what I think distinguishes OW from the other games.)
I can see it requiring some fairly different thinking and handling from the GM's side than your typical adventure where there's bad guys and you go find them and kill them. I haven't read a ton of the example adventure and what FF thinks constitutes an adventure of Only War....but part of me feels the game is almost designed to played like a weekly, on-going table top war game. At least, it's set up to do that and how you handle the action determines whether it ends up that way.
All that said, I think it has the best rationale for bringing a group of players together of any of the games, and the players being able to design their own regiments sounds really fun and like something people would get attached to. Only War puts far more control in the hands of the players during Character Creation, so they can really build themselves out to try and survive...instead of like Dark Heresy where characters start with jack and squat. Then again, the Only War book mentions "You" dying more than any other core rule book I've read through so far. While it's not stated outright, the implication to me is that OW characters are supposed to go in feeling expendable. (Since that's thematically what the Imperial Guard is all about.)