First, I don't understand why so much love for a death spiral system... the only thing like that in 3ed was level drain, and last time I checked everyone was rejoicing that level drain doesn't exist anymore.
A death spiral system as a default (meaning that it will happen to the PCs and against every single opponent) sounds a terrible idea to me. What is its purpose? If you want to scare the players (an exciting thing) it's best to limit death spiral effects to specific monsters, meaning that the PC will experience the spiral when fighting against them. If you want to warn them that it's time to run, low HP do just the same.
Otherwise, as a default you only achieve the result that the party who tilts the battle to its favor in the early rounds is going to accrue more and more advantages. Realistic yes, fun hell no! It means to always encourage the use of heaviest powers asap, and it means to make it more unlikely to see a battle that seems lost and becomes a victory.
Second, a death spiral system with a fixed condition table is IMHO too rigid to be fun at all, if every single time you're down 75% you get (e.g.) the same penalty to attack, then at 50% you're (e.g.) slowed, and so on... To be fun and interesting, it needs some variance, so that you cannot always know beforehand what kind of penalty you'd get. Too complex to be core, but would be nice as a variant in the 4e UA.
Anyway... I seriously think that there will not be a death spiral system in 4e. The "bloodied" condition will most likely trigger a benefit such as a special ability, eventually but not necessarily with added penalties (IMHO the 3ed Barbarian Rage is a good example). And I also think that the exact condition will not be the same for every creature, but each can have a different one. It might remain as a default for everyone to get the "blooded" condition, meant as being more/less subject to certain attacks depending on the abilities of those you're fighting against.