With the rumors flying around about 6th for next year has given some hints to new directions GW is taking this next edition. For the most part they are following a developmental pat that is taking them in the right direction.
Clearly they are looking at taking some elements from other games to work into the missions as an effort to balance them. By this I mean the rumors of statagems and bidding for first turn. Based on what I read, only a handful of the statagems are really good and worthwhile, so this means that while this will be a fresh idea it will get redundant quickly.
Also, decisions on combat will be more dynamic, to shoot or assult, no longer is there a middle ground with rumors the sequence being changed about. I feel also mechanized units will be taking the foreground even more so than ever.
Looking at the past trends of Codex books and the future Necron rumors. We see that the Necrons are getting vehicles, flyers, transports and new units. Almost a total facelift to where they started before. Making them a viable and no doubt with new toys a deadly army. The Grey Knights facelift saw a reduction in Inquisition feel and more to a core army of Grey Knights with allies on a smaller scale. Again, their impact in the game has been felt many times over.
Points are also shrinking for all current books, making games larger and creating a demand for more models for an army. I am sure this is by no means an accident but a method to push sales by adding more units to your army.
Characters in 40k also will be getting several boosts in 6th when it hits. Namely being able to claim 2 saves, being able to single out models in units among other nasty aspects. This is falling in line more like Fantasy.
Some armies will take a kick that was once king and unique like Death Guard and Blood Angels taking advantage of the only double save, Feel No Pain and their 3+ saves with some rare cases of Inv saves on characters and wargear. Now being able to take cover and armor saves will be a boon making armies tougher than before. Just at a recent tournament my Long Fangs was shot apart and that second save would have made the shift in my favor.