For those unaware, Faeit has some leaked pictures from The Gathering Storm II.
Ynnari faction lose the previous army-wide special rules like Power from Pain and Battle Focus. Regardless of whether those units had those special rules previously, ALL Ynnari faction units get the new Strength from Death special rule (the one granting extra actions when units die nearby). This means, among other things, that all Wraith units, including Wraithknights, get a pure power boost from joining the Ynnari faction.
You can seem to take almost any unit from Craftworld Eldar (Everything but an Avatar of Khaine, I think), most units from Dark Eldar (no Haemounculus, Mandrakes, Wracks, Grotesques, Talos, Cronos, Urien, Drazhar), and apparently everything from the Harlequin.
Ynnari factions can choose from their original codex's Artefacts OR from the new Ynnari Artefacts - kinda like the Farsight Enclaves (except the Eldar are still limited to only one artefact of any kind per model).
The Yncarne has a few interesting tidbits. It regains a lost wound on 3+ whenever any Aeldari MODEL dies within 7", has your basic MC statline with 3+, 5 wounds, and initiative 10, and grants a 12" radius bubble of Fearless and FnP.