This.
For me it comes down to core army mechanics. I haven't kept up to date on the 'humble proposal' thread, fyi. But let's look at army core mechanics in 6th, please jump in and correct me where I am wrong. CSM, supposed to work the challenge mechanic, you can see what GW was trying to accomplish here, and when it works it's pretty brutal, but it's clunky and difficult. DA core mechanic, stubborn and fearless on key units, with elite armies, both are considered middle/low tier I believe. Chaos Demons: core mechanic, gifts and Warp storm table, gifts are random and the warp storm table is a bit cray cray, mid to top teir? Who was next...Tau? Markerlights and supportive fire, both extremely good core mechanics, easy to leverage with no/little negative sides, top tier dex. Eldar: core mechanic, shoot and run or run and shoot, and negatives versus Slaanesh, who says fluff doesn't determine rules? Fluffy negative affect, but really doesn't come into play, fairly strong core mechanic, top tier codex. Space marines: core mechanic, combat squads and ATSKNF, both strong core mechanics ATSKNF is arguably one of the best USRs in the game, above mid tier codex. Tyranids codex: core mechanic: instinctive behavior, fearless within synapse range, most builds do not function outside of synapse range, not a strong core mechanic, actually punishing. Just some perspective