BoK has posted an update about 6th edition rules, stating:
Expect the game to be similar to 2nd ed 40k and 8th ed fantasy.
- Every unit gets a 6+ save vs. all Psychic Powers.
- Random Charge Lengths are in
- Pre-measuring is in.
- Random Battle field effects are in.
- All armies can purchase buildings for placement on the battlefield.
- Psychic powers are selected during deployment. (except Grey Knights)
- Deployment and Missions types have doubled.
Also, he says you can now allie (example Space Marine armies can ally with IG). I assume this means you can take units from other codices again.
You can read the full source here.
If this is like the change from 2nd to 3rd, they'll put a temporary army list for every race in the back of the large rule book. I wonder if they'll do a smaller one then at all, or force you to buy a new one to play. This would make sense from a financial standpoint, as those who aren't interested in the starter box still have to drop $60+ on a new rule book.