Here is what we know about the new patch from experience and discussions with the ZOS employees. I've added numbers to my points so that it's easier to reference in replies (just note what number point you're discussing). I can then update the next post with debates / arguments / etc.
1. On PTS
the new set items (see link for screen shots of them in the first testing phase - note that there have been some changes since) drop every reward mail.
2. When they go live, the set item drop frequency will decrease to the same it is now in reward mails.
3. You will still get a LOT of crap (deconstruct or sell) reward mails.
4. Sets like Sentry and Arch-Mage will no longer drop from reward mails.
5. We HOPE that the traits in reward mail gear will be primarily PvP-relevant (thus Impenetrable or Divines) and no longer well-fitted or sturdy (sturdy being the absolute worst trait for PvP as it's completely irrelevant). We acknowledge that some min/max damage builds want Infused on the majors (head/chest/legs) though those are few and far between.
Please see this spread sheet for information about how Impenetrable works, and if you still want a different trait then please tell me why - I'm serious, if there's a PvP scenario where you wouldn't want Impenetrable, I'd love to learn about it.
6. There are a few builds that may want a 3-set of Arch-Mage with Divines for PvE or Impenetrable for PvP, though for the most part very few builds should be using the old drop sets considering the stats and value in the new sets.
7. The new sets are already being theory-crafted and for the most part are FAR superior in mixes and matches to current sets.
8. The new sets come in ALL slots including jewellery.
9. You will be able to have two 5-sets active due to there being jewellery and weapons for ALL new sets.
10. You will be able to spend 100k gold or 200k AP for a PvP set item and a monster (undaunted) set item.
9. The items at the vendor per #8 will change every so often. Some people believe it will change every day, some believe every week. It may be multiple times per week. We don't know.
11. On PTS, all of the items sold by the PvP vendor come with the Infused trait. We have repeatedly objected that PvP items should come with a PvP trait (namely Impenetrable), though have not gotten a response from ZOS on that matter.
12. It is expected that the GOOD item set drops in reward mails (such as Vicious Death jewellery with the Arcane trait) will sell for a LOT of gold in the first few weeks.
13. Groups that are at most 6 players will make double the AP as they used to make. Larger groups will make less.
If you want details of how AP works in the new (and current) patch, look at this link.
14. Due to these changes, it is very likely that you'll see most experienced players cap their groups at 12. Most won't want more than 6, though will grow at times if required.
@ZOS_BrianWheeler stated in one of the guild meetings about a month ago that they want the number of combatants required to defeat an opposing group to be roughly similar. There should not be a situation where two people are able to kill 20.