Gunship (you'll barely notice the difference) -> Marrowgar (3 healers) -> Lana'thel -> Deathwhisper -> Saurfang -> Rotface -> Festergut -> Valithria -> Blood Prince Council -> Sindragosa -> Professor Putricide -> Lich King
Gunship you'll be hard pressed to notice any difference really from normal mode than heroic which means in terms of difficulty its a no-brainer to be first. Marrowgar isn't too bad with 3 healers, if you can get a 4th healer its a faceroll. Just takes longer with the more healers you can bring or get people to offspec for. Deathwhisper is not much more complicated than normal mode. Just a case of timing the transition to P2 and people avoiding damage from ghosts and proper CC. Saurfang continues to pretty much be a DPS rush before you get more marks than your healers can cope with. With the exception of the adds needing nuked quicker and some slows, its pretty much the same fight but more damage on the tanks and the marks.
Rotface just has vile gas added to it so providing people are spread out it won't really affect the encounter and its pretty much the same thing - the ooze pools do apply a slow effect so there is less kiting room for the tanks. Festergut just hits harder and there is malleable goo to contend with. Prof putricide is probably the most demanding encounter (outside of LK heroic) in terms of DPS, healing and organisation and transferring the disease. There are loads of methods out there for handling the disease, work out which is best for you.
Valithria is basically the same encounter but the healing output needed is higher and control of the adds needs to just be good. Sindragosa is probably second hardest encounter as a result of the extra mechanic placed on your casters and healers. Its a lot more movement needed during P3 for many people which means its probably more challenging than most of the others. You'll barely notice the issue until Phase 3.
Blood Prince Council is made tricky with the fact that you'll probably need 3 healers, 2 tanks and 1 of your DPS acting as a tank. You'll need 2 ranged to focus on Kinetic Bombs which can spawn anywhere and drop fast so need a lot more attention. Basically means that the majority of your damage is going to come from 2 dps only which means that the fight lasts a lot longer. Lana'thel is basically the same fight as normal with the second air phase needing a bit more healing on it is all but providing people are spread its exactly the same.
Thats certainly how I thought the difficulty scaling of the encounters worked.