The two things that strike me are the 5 missions (which have cropped up from time to time as a 6th Edition change) and allies.
I was laughing pretty hard at the Tau going under Astartes protectorate, then I saw that official event alliance grid and couldn't help but notice that Spehhs Mhreens are "brothers in arms" with Tau. And that made me pause.
I... Am not against it, I mean, I'm okay with fluff not being set in stone and I've never really taken it very seriously in the first place (it's hard to), and if this means allies get back in then hoo boy am I excited, it might actually compensate the utter lackluster, partial nonsense and immense boredom we got with the Sisters' WDdex, so maybe I won't give up and start (probably) collecting Chaos if the new dex doesn't suck. Though knowing myself I will probably get a model or two regardless.
I frankly never understood why they did away with the allies rules, if it was such a bother in competitive play just make it unavailable at tournaments and bam. I also don't get why they still make people pay for a codex, especially at such prices, instead of making them available for free online and make physical copies optional; if more people know the rules, especially those specific to each army, you have more people that can potentially be interested in playing and by extension buy miniatures.
But I digress, and I apologize for that.
But hey, maybe this Tau and Space Marine suddenly becoming buddies thing means that Matt Ward will do the next Tau codex?