Let me start by saying that you won't be disappointed. I admit I was afraid things were going to be pretty sketchy this season, with the show's creators taking a leap of faith by transplanting the series a year into the future. I'm extremely pleased at how well it's working, and there are lots of hints of promise for what's to come.
The opening scenes of the two-hour premiere (called "Occupation / Precipice") are almost poetic, and they definitely give you that excited feeling that "holy crap, BSG is back!" That haunting, ethereal music, teasing shots of people we've come to know on the show. It only keeps getting better.
Now, for the spoilers:
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We start about 134 days later than we last left the show. I'm reasonably sure this isn't news, but I can now confirm it. The colonials have been living amongst the Cylons for all this time, and you'll see all that's changed because of it. And I think there are things that will surprise you.
Someone who many suspected to be a Cylon last season looks not to be one. It's not explicitly stated, but when you see just the opening minutes of the two-hour season premiere, you'll likely come to the same conclusion.
Starbuck's situation will surprise you. Not only will her whereabouts surprise you, but when Starbuck meets someone named Casey, you may be surprised at her reactions toward her.
Lee Adama's state of body and mind is a bit shocking. Remember, he's back on Galactica and Pegasus with his father and a paltry skeleton crew. He hasn't had a whole lot of motivation, to say the least.
Colonel Tigh isn't taking things laying down. Maybe it's just me, but I've always felt Tigh acted very much like an old seasoned sea captain, who'd take no crap from the bad elements thrown at him. I was disappointed when we saw a lot of his weak side on Galactica in the past two seasons, especially when he couldn't do a whole lot without Adama around. He's acting more like that seasoned sea captain now, along with the battle scars to earn it. Put it this way -- the promo pictures for season three showing Saul aren't quite accurate (yet, at least).
There are dogs on New Caprica. Not really a spoiler, but I found it odd that there was a domesticated dog on the planet, when they've only been there less than two years. Did they really have dogs on Galactica?