The personalities that make up the Imperial Guard are as infinite and as varied as the human race, the possibilities are as limitless as human nature itself. You don't get this with Space Marines, at least not to the same extent. The regiment will play a large roll in shaping the identity of character but your much more likely to find the full gambit of human emotion in the Guard than you are in the Astartes. With pride, stupidity, determination, cowardice, anger, fear, greed, heroism, jealousy and even treachery colliding within the same squad, let alone the same company or regiment. I think the potential for diversity will be a major attraction with sweeping campaigns featuring Armoured Regiments of every stripe, Storm Troopers, Rough Riders, Drop Troops, Aeronautica (Anyone fancy Voxing in an airstrike on that Ork compound?!) not to mention all the various worlds and battlefields you could fight across, perhaps requiring a Battlefield Compendium might be cool?
I don't want to slag off Space Marines (I'm not that dumb!) but Imperial Guard are just better and offer so much more potential for RP, the chain of command is an excellent DM tool, if you think its an obstacle to RP then thats what it'll become, but really its another avenue for plot hooks. Such as your squad being given all the worst missions because your CO dislikes one particular grunt in your squad, I'd love to try out that dynamic. I'd also try putting my grunts in space in a Naval Battalion having them boarding ships, space stations and orbital defence batteries as well as putting down riots (in fact putting down a riot plantside could be an awesome intro to a defensive campaign)
"Your eternal soul may belong to the Emperor, but your worthless hide belongs to me."
Gunnery Lt. Schenn (Navy), aboard the 'Manifest Destiny'
The real difference is the human face on things.
The Adeptus Astartes are His Angels of Death, they shall know no fear. A Space Marine's job is to take on the biggest and nastiest beasties in the galaxy, and use his amazing training with the best weapons possible to triumph. He hates the mutant, the heretic, the xeno. He knows his role as a living weapon, and thrives in that role.
Imperial Guardsman are conscripted en masse. They're stuffed into flak armour and handed an M36, and then they're shipped across the galaxy to fight things he doesn't understand. He misses home and his family. He'll likely spend a lot of time in a muddy trench. When the Carnifex shows up, he'll probably piss himself in terror or run forward because the alternative is a bolt round to the back of the skull. Guardsman are average joes: farm boys and hive orphans and manufactorum workers. They want to smoke lho-sticks and eat something other than corpse starch for once. They want to spend a night with the ladies in town. They probably want something thicker than flak and faith to keep himself safe from the claws and bullets of the enemy.
It's the difference between Superman and a kid taking ROTC, except they're both fighting the same bad guys, bad guys that can and do kill Superman. You can tell a lot of stories with that. An Astartes sees a populace turn to secession or The Greater Good, and sees only weakness. He kills them and probably feels little. A Guardsman sees a secessionist and he understands that it really, really sucks to have the only Ecclesiarch he's ever seen preach about the good of the working class and then have an assassin sent after him by the very Imperium he should love. He hears about the Tau offering a life based on working together, where all toil equally instead of having nobles spit on him, and he probably sees why that's a good idea. He probably thinks it beats having a Commissar point a bolt pistol at his back all day!
I've converted Dark Heresy to the Only War system. Please take a look!