ADDENDUM:
This is a post I made earlier (there a lot of threads identical to this one), after doing a bit of research.
I make no claims to the accuracy of this information, since as we all know, the details of a game are subject to change in the creation progress.
If you don't wish to know anything, stop here, consider yourself warned.
And without further ado, here is what (we think) we know:
It takes place in the Helius Sector of the Andromeda galaxy.
That means that from a gameplay perspective, the Mass Effect trilogy will take a backseat, given the enormous distance between Andromeda and the Milky Way. Cameos and references are quite likely, but aside from the base lore (different races existing, the "Mass Effect", eezo, etc.) it will most likely be something distinct from the original trilogy.
There is a character named "Ryder".
Whether he/she is the protagonist (aka "the Pathfinder") remains to be seen, but I think it's likely, given the BioWare tradition of giving the protagonist a NAME-TITLE for the player character, especially if voice-overs are present in the game.
There is a ship known as the Ark.
We don't know if the Ark is going to be similar to the Normandy in function (It appears to be too large for a stealth frigate, if appearances are anything to go by, it is presumably a colony ship. In all likelihood it's more akin to a space fortress; with an name like "Ark", it must be a very self-sustaining ship (much like the biblical Ark), but with weapons.
Resource infrastructure and settlement establishment is important.
It looks like we have to balance things on a societal level; I imagine a ship like the Ark requires lots of resources, so securing additional ones looks to be a good source of side missions.
There will be a "Strike Team", that you can send out to collect resources, defend outposts, etc.
I get a "War Table" vibe from this. I liked that feature, and hopefully it gets improved upon here.
There are seven companions. You get to customize/upgrade their weapons, gear, and even appearance.
One of them is a biotic named "Cora", and a Krogan named "Drack".
There are other characters that can be befriended/romanced that don't necessarily follow you in the field, akin to Cullen or Josephine from Inquisition (you can befriend/romance them, but they aren't companions in the traditional sense.)
A bit small compared to other BioWare greats, but seven "selectable" is still plenty to work with, so it's reasonable to expect a balance of both character archetypes and gameplay capabilities.
Loyalty missions return.
YES! I loved this about ME2, and it doubles as extra content, character development, and loot. A triple-win.
There is a faction known as the Remnant, whose tech and equipment is psuedo-Prothean in nature (ancient, ultra-powerful, etc.)
Extra lore is always nice, and it appears to be a priority regarding the aforementioned resource acquisition and settlement.
There is a villainous faction known as the Khet, companions react favorably when you disrupt their influence in the sector.
I like it when you can do something that everyone approves of, like blowing up Cerberus in the Mass Effect Trilogy. It appears to also expand the narratives you can pursue with them.
Dialogue is more intuitive than ever.
Interrupts are expanded upon, with more options to change how conversations go. I get the feeling that the Paragon/Renegade system is still present (I hope it is, it's an excellent system) but with more options within them.
We are not alone.
This isn't an empty galaxy. Forging alliances and working with other groups in paramount to survival. It looks like some of those alliances won't be easy, it will also affect companions' approval.