Ork and Eldar fleets are in the game at release. So far the only DLC mentioned is the Space Marines, which I think is quite acceptable because Space Marines are not that big of a player in space battles, being much more of a ground assault force.
Edit: I'll also mention that the free Space Marines DLC is extended for 2 months after the game is released in case you want to see reviews of the game first.
From the video; You can rename ships, ships have customisable abilities, upgrades, crew and favours with different factions (Imperial Navy favours have; Inquisition, Imperial Navy, Adeptus Mechanicus and Adeptus Astartes), ships gain experience individually from battle, ship commanders can flee a battle if they lose moral giving you the choice of executing them. There's also several different mission types including fleet duels, space station assault and planet assault.
Edit: I'll also include that in the campaign mode, your fleet is not the only one around as there will be A.I. Imperial Navies that you can work with to defeat enemies and defend the Gothic sector. In the video you see one area, of which I think they mentioned there is about 6 total.
There's also a number of interesting mechanics during battles; bombs you can place to try and catch enemy ships, stasis bombs that slow time down locally affecting shots and ships, boarding manoeuvres, broadsides with torpedos, fast turns, ramming, a locust swarm for a Nurgle marked Chaos ship. Space bar also gives a kind of pause with it slowing time down to help make decisions during battle.
You can also give ship control over to the A.I., telling them to engage at different ranges, port, starboard or bow focus for weapons.
Planned features are the other factions (Tau, Necrons, Tyranids) and a ship painting system as a free update post launch once it's finished.