Ok, so we find out:
- Why the plane crashed
- Why they need to push the button
- How Desmond got to the island and hatch
We are left with loose ends regarding:
- Jack, Sawyer, and Kate
- Sayeed
- Locke and Eko
- Desmond
- Michael and Walt (sorta)
We are also given new mysteries:
- Why didn't Desmond manage to sail away from the island?
- What's with that 4-toed statue foot???

- The invisible ink map that ex-army guy was drawing - why was he drawing it, and how did he know what he was drawing?
- With the revelation that the notebooks with all the written observations get tubed out to the middle of Tarzan's backyard, it does seem that the observers themselves were the ones being tested. Why?
- What is causing the electromagnetic buildup?
- What happened with the hatch and all that equipment? "Violet glow in the sky"???
- How in the world did Desmond's girlfriend know to set up a monitoring station that would look for an EM anomaly???
Along with a number of old mysteries (I probably don't remember them all):
- What happened to the black smoke monster thing that we haven't heard a peep from since Eko scared it off?
- What was the quarantine hatch, those vaccines, and (new) those isolation suits protecting people from?
- And of course, why are all the metaphysical things happening on the island (Locke being able to walk, the black lady being healed, people seeing other people from their past)?