Warhammer 40K Tactics V: Everyone Is On Fire. Some Moreso Than Others.

[Cheesegear]

At least Soul Grinders appear decently awesome, judging from their capabilities...

Soul Grinders are just about the only thing guaranteed to be effective. For one turn. Then they get shot at.

Some of the Greater Daemons, Keepers of Secrets in particular, look quite effective, too.

IMO, the Keeper of Secrets is the best one. There is no special character KoS because it's not really possible to make a KoS better than it already is.

And anything riding a Khornate Juggernaut should tear into the enemies like a hot knife through butter.

Yep. Therein lies the problem. All the good units you've described are Heavy Support, HQ and Elite. Chaos Daemon Troops and Fast Attack are kind of really bad.

Largertyler:
What do you need Creed for?
No Heavy Weapons? Granted, it is 500 points. But, you still need S7 or 8 weapons to deal with Dreadnoughts. Think about trading some of those Grenade Launchers for Missile Launchers.

...That's about it. It's only 500 points. There's not much to say. Given that you've done it right, your Imperial Guard list should pretty much win every game. Except against shooty Space Marines and Tau, who will cut through you like butter.