Perhaps a little early for this kind of thread, but here goes anyway.
For those unaware, the 8th Edition Fantasy rulebook did a great deal to re-balance the game overall. New mechanics and rules helped to tone down the nastier armies, whilst buffing up the more tired armies. The result, even before the new raft of Army Books began hitting was a much better game overall, with less of a power gap between certain books.
And it would seem they are attempting the same thing with 6th Edition 40k.
I'm going to approach these changes based on the complaints I've most often read about online, so it's far from exhaustive, and remember that at all times this is just my opinion.
So, off we go.
1. Feel No Pain. Confirmed as dropping down to 5+. Phew. This became fairly prevalent in 5th Edition, whether through psychic powers, or mass upgrades to squads. To those armies with wide access to FnP, players often depended on it, leading to a great amount of frustration from opponents. Whilst not exactly game breaking, it could mean players weren't suitably punished for silly deployment etc.
2. Overwatch. It's about time if you ask me! I've never really played HTH heavy armies, prefering my short ranged firefights. Having played through the ignominy of 3rd Edition, where a squad in combat basically stayed in combat, through the various toning downs, this is the iceing on the cake for me. No matter what my opponent tries, my guns can now give it some dakka, which is nice.
3. Fear. Barely hinted at in White Dwarf, it's noted that all Daemon units cause it. Exact mechanic remains unknown though. But regardless, it's about bloody time. In Fantasy, when a Greater Daemon, or something akin to a Hive Tyrant comes barrelling at your troops, there's a fair chance they'll head for the hills. But ever since 3rd Edition, even a lowly conscript has simply given a round of applause as some colossal horror devours the command squad. Psychology has been sorely missed in 40k, and it's good to see it back!
4. The different valued objectives. HALLELUJAH! Now THAT is giving the game tactial depth. I loved 5th Editions victory conditions when it came out, but sadly as all objective were equal, my tactics stagnated. Deploy my objective on homesoil, set up a bunker unit. Sally forth to drive my opponent off his. Rinse and repeat game after game. Now though, I have to try to second guess my opponent, and he has to guess what I'm up to. Base sitting may not work all that well now, and it's possible the most valuable objective will be somewhat exposed. Small change to the rules, big impact on my tactical nous.
Now all these things I see as being overtly positive. It's hard for me to go into much more depth as so many of the changes remain largely unquantified! Now, over to you. Ideally, as there are many threads discussing concerns, try to stick to what you see as positive changes in this one!