By request, here's the tactica for all things diseased, bloated, scabby, and/or infested. We have a tactica death guard, which though it's near and dear to my spleen, I think we could use something a little more broad.
I'll start things off by throwing out a few ideas, and see where things go for there...
First off, for codex daemons, I've got some thoughts on playing mono nurgle. Right off the bat, I'm going to say I don't see epidemius as worth taking. Sure, I'll throw him into an apoc list, especially if there are other nurgle players present for gits and shiggles...but for a straight, by the codex daemon army that I'd like to win games with. Nope. Two great unclean ones are they only way to fly (god I wish they could fly) in my opinion/experience.
I go for a daemonzilla approach, with the two GUOs, and three daemon princes of nurgle as the core of my army, and fill in points with plague bearers. Nurglings are cute, but ultimately no better than plague marines, and beasts are...well, difficult to see the value in, I'll put it like that.
Having five nice big monstrous creatures has both a mechanical affect, in that small arms fire really needs to be poured into me to get my attention, as well as a psychological affect, in that that's a lot of blubber that people need to wade through, which is demoralizing. Cloud of flies is manditory, as is flight on the DPs. Breath is nearly manditory, as swarms of infantry, heavy or light, will melt away before the rancid garlic breath of my nurgle collosi. Beyond this, I don't look much further for the GUOs, and normally like to keep the princes light as well. Noxious touch has it's merrits, if there's points to spend.
The rest of the army is in essense, x number of plague bearer units. Whatever it takes to fill in points. Normally speaking these guys are my second wave, so they can plop down on objectives while everything large and sweaty is being focused on. If the MCs have been charged, the hand to hand can be used to block line of sight on PB squads, which can charge in next turn and add their weight to the melee.
Beasts, I'm still experimenting with. There really isn't much you can do with beasts you can't do with plague bearers, but you can only have so many plague bearer squads.
For codex chaos marines, I run a mechanized deathguard list. Depending on who you ask it might be seen as fluffy, or not. I don't particularly care, as the storyline for warhammer is...well, juvenile...in my personal opinion, and largely unimpressive.
Anyways, following pretty standard chaos player protocal, the force is lead by two daemon princes. Depending on points they will either be spartan style (Sanctjud's pattented wings only variety), or winged, marked, and either warptimed or rotted. Often it ends up one with warptime, one with rot. All in all it doesn't matter, as I'm notorious for forgetting psyker powers. I haven't gotten a chance to try it out yet, but I've been contemplating two spartan princes used as mobile cover for my rhinos...which is opposite of how I normally run things. If I get an opportunity to play any time soon I'll have more to say on it later.
Just about the only other thing in my army is the troops. And their transports. Traditionally I run two to four units of five plague marines, with two meltaguns. Champ with combimelta, sometimes champless. These squads are in rhinos. No frills, no pazaz, just tough SOBs with a pair of anti tank guns. After this we have two units of eight or so plague marines with two flamers each, and a champ with combi flamers. They ride in landraiders. This is my two giant middle fingers of doom to orc players everywhere. In my neighborhood all you see (fungal wise) are huge mobs of boys, and some nobs with a boss. Three flamer templates, and a charge of 24 or so attacks, and there's a heeping pile of dead orks. This coupled with the fact that orks can't handle landraiders for squat, makes me very very happy.
Possible additions are lesser daemons, as these are the ultimate utility unit in the codex, and obliterators, which are the ultimate in offensive codex units. I've talked at great length (as have others) over the great things that can be done with lesser daemons, and I'm too tired to repeat myself right now. As for oblits...well, there are plenty of places to tell you how to use your obliterators...