Gotta be honest here, my army looks the same as the first with all carapace. Firstly make one of those Leman Russes into an exectioner and it does wonders. Secondly, yes, there are very valid arguments for not taking carapace but I have to point out that a majority of basic weapons are AP 5 these days(bolters, pulse rifles, etc). Dont take stormtroopers, they suck horribly. Those sentinels need to have autocannons badly for the sole reason of a guardsman's OK ballistic skill. Lascannons will statistically get 1 1/2 hits where as autocannons will get 3. If you doubt anything I say look at my battle record.
93rd Saphra Armored Regiment: 128-101-13
Iron Warriors Warband: Destroyed
Raven Guard 6th Company: 29-19-7
Black Templar Reclamation Destroyed
Tau Bora'kan Hunter Cadre: 14-10-2
Unnamed Adeptus Mechanicus Force: Coming Soon!