So I built my first deck last night using some of the Shadows cards.
One of the first things that jumped out at me was the increase in cards used during set up. First, attachments and events that otherwise would be unavailable for setup, are now a possible drop if they have the Shadows crest. Second, Shadow cards that have a total cost of 3+ can now be dropped during setup for 2 Gold, with the rest being paid to bring the card out of Shadows.
This is just a fairly minor benefit of Shadows, but it really jumped out at me, particularly as it wasn't something I had thought of when doing my first analysis of Shadows.
Until I see the rest of the Shadows cards in the next few chapter packs I can't say this for sure, but I definitely expect Shadows to have a major impact on the Championships at Gencon