The 6th edition is upon us and while it seems that there will be e new CSM codex out fairly soon, we still have some time until then to look at how current Chaos fares with the new rules. As a DG player, I will mainly concentrate on nurgly stuff, but it seems that in general the gods were merciful on their followers.
Good changes from a Death Guard perspective:
Thoughness 4(5) counts as Thoughness 5: Probably the biggest change! Infantry and characters are more resilent and bikes zoom around with a T6.
FNP: At first the reduction to 5+ seemed like a down side. But FNP now works against everything but instant death. Together with T5 plaque marines now have FNP against all but Railguns and Dreadnough CCWs.
Rapid Fire: Infantry with rapid fire weapons now have a greater threat range. Plasmaguns seem more useful now, as they can glance a vehicle to death.
Overwatch: Nice addition to thin out those attackers.
Allies: Daemons are back. We will probably see quite a few Epidemius + DG lists. Epidemius abilities seem to work on Plaquemarines too!