(See my previous post for the codex discussed here)
First off I want to mention a few changes I made to my initial post, due to some great feedback from sonsoftaurus. I added the missing Typhus - with the Cloud of Flies rule and a 10 point price raise. The rest of the changes were to Plague Zombies. I clarified that they are destroyed when in reserve if there is no Zombie Lord for them to be raised by, and I added the missing Join Us! rule. I also removed the ability for Plague Zombies to score. This was done partly because it made sense in the fluff, but mainly to help differentiate their role on the battlefield with the much more expensive Plaguebearers. Now they are more of a throwaway unit which was how I imagined them being used.
Now let's start with the HQs. There are two important issues with the Prince/Lord/Sorcerer: upping the cost for feel no pain, and the removal of wing/bike options. Without raptors or bikers being included in the army it made sense that the commanders would also stat on foot. Losing wings from the Daemon Prince should counter any complaints about the addition of Feel No Pain. I charged the Lord and Sorcerer less for FNP because they can't be picked out of their unit for shooting so it is less valuable than on the prince.
The reason for reducing the cost of the Great Unclean One was because you need to kill a champion to bring him in to play, and that he must enter play through reserves. I instead built his cost up from that of the generic Greater Daemon in the CSM Codex.
I had to think a bit when deciding how to handle Lesser Daemons. I wanted to use the same rules as those in Codex: Chaos Daemons (to avoid confusion), but I wanted to avoid copying half that codex, without forcing the player to purchase a second codex to play their army. So I compromised - I included Plaguebearers, who feel iconic to me, and skipped the other Nurgle themed Daemons.
The decision to make Rhinos take up a fast attack slot was motivated by fluff, not balance. The Death Guard prefer to fight on foot and have little motivation to maintain their vehicles. This won't have much effect smaller games, but will force difficult decisions when building a larger force. With nothin else besides Spawn to select anyway it won't have s large impact.
I encountered a lot of resistance to including heavy weapon squads in a Death Guard force, with claims that it was unfluffy. I dispute this - sure they don't keep heavy weapons in basic troop squads, but I'm sure they have squads dedicated to long range support. My compromise is the Pandemics squad, able to take a couple of lascannons to make an all-infantry army at least feasible.
I made the decision to make the Daemonic Possession rule mandatory across all vehicles in the army. This represents the fact that the Death Guard do not spend time on maintenance - the only reason they are still running is because of the daemons that have made themselves at home. To this end I even added the rule to the Chaos Dreadnought, despite it not being an option in Codex: Chaos Space Marines. I also changed the terrible Crazed! rule to one with a Nurgle flavour. The new rule means that your Dread will only shoot at you enemies - but it also means they will only shoot two thirds of the time. Sure I have made it better, but that's only because it is so crap currently. This way it becomes a viable option.
The Vindicator is not a pre-Heresy vehicle, so the Death Guard should no have them in their arsenal. But about of Chaos players have probably picked up a couple for their army - and the concept of a short range vehicle really suits the nature of Plague Marines. I wanted to include an alternative use for the model, so the Regurgitator was born. This is one of very few vehicles in the game that have the ability to wipe power armoured troops in cover off the board. I based the profile on the Collosus - and the improved armour and possession of the Regurgitator is balanced by the short range and additional cost.
In general I tried to avoid adding army wide rules that would require extensive playtesting. The only two needed were Cloud of Flies (which I just borrowed from Codex: Chaos Daemons) and the plaguesword. This weapon wounds on a 2+, making it great against monstrous creatures and hordes but less effective against marines than a power sword.
Hope this helps shed some light on the motivation behind the design decisions in the 'dex. So anyone out there tempted to give this a go?