Avacyn, Angel of Hope poses a huge problem for most decks. At eight mana, three of which being triple white, you won't see her descending from your heavenly hand to the battlefield in most standard decks.
Instead I think players will opt to place her into the graveyard and then use Unburial Rites to reanimate her out as early as turn three. Imagine, if you will:
Turn 1 - Land, Birds of Paradise
Turn 2 - Land, Faithless looting (discarding Avacyn and Unburial Rites)
Turn 3 - Land, flashback Unburial Rites. You have an indestructible 8/8 with flying and vigilance that also makes everything else you own indestructible.
There will be other ways for getting this angelic guardian to appear on the battlefield but I think that this will be the most common/popular method.
Another problem with regard to Avacyn is that there aren't many ways to get rid of her however; she can be dealt with. If you're in a position to hard-cast her be wary of the ever frustrating counter spell. Liliana of the Veil's sacrifice ability or Tribute to Hunger would also disrupt your saviours strategy.
Of course, we don't know what else Wizards have in store for us. Previews officially begin next Monday on the daily mtg site. (Link can be found on the gambit blog)
Until next time,
John, Angel of Hope