for aslong as i can remember their has allways been a power creap in 40k the latest books were allways the best and their fore created a gap between these codex's and the older ones.
but with the arrival of 6th ed their has been no power creap the 6th ed codex's by them selves are not powerfull at all their very balanced and nothing stands out as an auto choice for a strong build.
and here is why!
6th ed brought allies and with allies comes a bigger picture the new codex's are no longer stand alone books their supplements to other armies so the realistic way to read a 6th ed codex is to see which army you have that you can fill with extras to boost your army. its not that these codex'scant function as a stand alone codex they can and will but i believe that 6th ed is now all about allies and bringing new combos to the table. i also think this will carry on with each new codex that comes out OR ill be very surprised if they write a power dex that can dominate everything else
so embrace allies and see wat it can do for you!!