Actually picked up a couple of the theme decks yesterday, and I was surprised when I sat down and played them at the synergy that was actually present in them, Nightfall especially.
Nightfall is all about discard pile abuse. Sage's Training and Mawile give you extra cards while strategically discarding cards off the top of your deck, cards that can be recovered with Lairon's Scrap Attack, Sableye's Pull Out, or Burned Tower, or feed Honchkrow's Vengeance. Metagross gives you a big hitter in your secondary energy type, and Skarmory and amusingly enough the Umbreon line are filler to pad out the deck, though Skarmory is potentially very useful if Special Metal Energy is added.
Daybreak on the other hand is meant to be much more of...I guess a control deck? You have a lot of healing (Bellosom, Togekiss, Espeon), Dodrio as a bench sitter to give every Pokémon in the deck free retreat, and Team Rocket's Trickery for disruption. Oh, and some special condition abuse there as well. And Espeon actually works well as a focus of the deck, Solar Suggestion is a very impressive damage shift for 1 energy. It does have a weakness in that its biggest attacks are either conditional (Muk's Pester) or flippy (Bellosom's Dance 'till Dawn, Togekiss' Air Slash).
That's not to say that they're at all competitive, but they are good for showcasing the sort of strategies that are possible in the set, and are excellent for bringing new players into the fold. Getting a Sneasel and Special Metal Energy in Nightfall's booster and another Espeon in Daybreak's helped too.