An anonymous source (who supposedly has the hookup) answered a ton of fans' questions about the first two episodes. With a slight pinch of salt, here ya go:
Among other things, the island is traveling backwards in time, but its time travel is unstable at first, so you will see short snippets of different time periods. And Daniel Faraday is able to do something in the past that affects the future. We see a past Dharma station, but don't see the island pre-Dharma.
The crying baby (from Marvin Candle's video shown at Comic-Con) puts in an appearance. Ana-Lucia appears as a ghost/vision (to Hurley) but not on the island. We don't see Desmond, Penny and their baby all together, at least in the first two episodes. Daniel and his raft make it back to the island, but we don't see the Black Rock arriving on the island (in the first two episodes, anyway) or the Oceanic Six getting back. We see a flashback of Kate on Penny's boat, thinking about Sawyer, but there's no romantic vibe to it.
The following characters don't show up in the first two episodes: the smoke monster, young Ben, Radzinsky, Frank, Cassidy, Danielle, Tom, Christian Shepherd, Nikki, Paulo, any dead characters besides Locke and Ana-Lucia. The opening scene of episode one is shocking only because it's happening on the island.
We don't find out who's behind the "flaming arrow attack," but it's not Rousseau's people, and it's not the Hostiles. It may or may not be the Black Rock crew. Neil Frogurt dies during the flaming arrow attack. Also, "The Hooded Character" writing on a board is Miss Hawking, and she's on the island. [SpoilersLost]
In a slightly more reliable set of spoilers, Empire Magazine visited Oahu and talked to people. Producers say the new season is about what happens if and when the Oceanic Six get back to the island — and time is not what we think it is. "Like, if you took string theory, tied it up in a huge knot, and threw it up in the air," says producer Jack Bender. More concretely, you'll see scenes and have to figure out when they each take place, and how they connect up. Or put it another way, the island is like Narnia, while the real world is like Britain — time runs at different speeds in each place.
The new season is "more than usually complicated," warns actor Michael Emerson. "We are all over the map in a temporal and geographical sense. [Empire via SpoilersLost]