After watching Team covenants demo video I started making short list of things this game does different. Campaign mode of course, skirmish mode is totally new kind of animal.
1. No overlord deck. In place of that you get resources to buy new squads, it also replaces reinforcement mechanics.
2. New results on defense die - - one that lets you cancel all crits and one that lets you dodge attack entirely, but only white defense die (dodge type) has that one
3. You can only spend 2 strain (stamina for initiated) for movement each turn
4 New skill mechanics - only 3 skills compared to 4 from descent, and checks are done rolling normal attack die, if you get at least one surge you pass
5. New rest and healing. If you do rest action you remove amount of strain equal to your endurance value, if you had less strain compared to that value, you remove the diference as health, so now you can recover health with rest action.
6. Scaling to group size is done now by giving overlord more creatures, but by giving player characters more activations (still not sure on that, but seems that way from smugler char sheet in main FFG page)
7. players and overlord move one character or group at the time so less downtime.
Comment if you see any mistakes I made or find new info