I've seen the demos and it seems all tactical shooter/melee. How will the force be handled and what effects will it have on the game and its mechanics? Anyone know??

The playtesters know.

But in all seriousness, I imagine that in the campaign mode, Diala will have cards in her personal deck that she can purchase to give her Force abilities. I could imagine a Mind Trick or a Force Push ability to stun or move enemy units at the cost of exhausting the card and/or taking strain.

For the skirmish mode, it's hard to guess because we still don't have a firm grasp on how it works. But at the bottom of the Description page, you can see a card called "Defelction" with a subtitle of "Force User." It seems to me that you have a deck of deployment cards similar to BattleLore's command cards and you take turns playing deployment cards from your hand. These will dictate which units you activate such as "Force User" on the "Deflection" card or "Brawlers" on the "Close the Gap" card. So it seems that we'll see more Force-abilities as deployment cards in the skirmish mode.

[Disclaimer: I am not a playtester.]

My guess is that the Force will closely mirror magical abilities from Descent. The characters have built in abilities rather than a spellbook. I wonder if the cards are for the Imperial player as overlord-esque abilities.