The Astra Militarum as I have been hearing is more and more the same codex we had, without the Ordinance Batteries. It seems that the biggest changes appear to be with orders, the Tempestus Platoons, Tank Commanders, and the new Taurox.
Please remember that this is part 2, and should be considered rumors until we have an official release. These are from someone that has read through the codex, and in his own words......
the codex hasn't changed too much but after i'v read it all it's good enough and let the IG be more aggressive
A link to part 1
http://natfka.blogspot.com/2014/04/from-codex-astra-militarum.html
via an anonymous source on Faeit 212
Regimental Advisors are the same
Tempestus Platoons are an elite slot and must be formed by 1-3 scion squads + 1 tempestus command squad.
Ratlings are the same at 30pts
Platoon Infantry squads are the same and can still combine
Penal Legions are gone
rough riders at 55 and two can take 2 special weapons.
All the Hellhound variants are there, and the same.
The armoured Sentinel is 40 and can take plasma. Scout Sentinels are still 35 but get scout and move through cover and cannot take a plasma cannon.