Korlandril here, and I will be looking at several changes within the fluff in the new 6th Edition rulebook. This is by no means a comprehensive list but, it highlights a few changes I found interesting.
The 6th edition fluff section numbers around 100 pages which is 20 extra pages than the last edition and 30 more than 4th edition. In my opinion GW have done a good job in fleshing out the section and it is still intriguing to read even after reading essentially the same story multiple times.
Another interesting addition I found was within the timeline on page 171, it mentions The Redemption Crusades where, although it does not say specifically, that the five Primarchs returned. This raises many questions, "could they return again?" being the main one. Although its clear who 4 of them are, Vulkan, Leman Russ, Jaghatai Khan and Corvus Corax as these guys have never 'died'. The fifth Primarch could either be Lion el Johnson or Roboute Gullimen. My bets would be on Johnson as only the Watchers, I believe, know of his continued existence whereas Gullimen is in a stasis field. Maybe GW will bring out models/rules for using these five if they return again ? (Apocalypse only of course)
A final change I found interesting, being myself primarily an Eldar player, is the mention of 3 new Craftworlds I have never come across before, on page 205 in the third paragraph. These being Mero, the doomed Craftworld, Chto, the legendary lost craftworld and finnaly Drohai, who refuse contact with all other craftworlds. This would definitely be a sign of the new Eldar codex, fluff wise at least, being completed as the way it is written would suggest it is in the codex.
Please tell me your opinions and other changes you have found interesting.