Firstly the questions then:
What is the significance of the symbol containing the note (or list) in the guitar case Hurley was carrying? Has the symbol been seen before in the series?
We've seen through flashbacks that Hurley's bad luck derived from him winning the lottery. In the "flashsideways/alternate reality" he still wins the lottery but is now "the luckiest guy alive." What gives?
Desmond's appearance on the plane cannot be seen as merely coincidental. We've heard already that the rules don't apply to him, he must have a motive, albeit one that we can't possibly begin to understand until later in the series. The question is should his disappearance be seen as done through supernatural means as we don't see him leave the plane. Admittedly this holds flaws as we didn't see characters such as Claire leave the plane either.
And now onto the nitpicking:
If the pan from the airplane window is to be believed then in the alternate reality the island is not "1000 miles off course" and near the coast of Fiji. Although I can accept this as a plot hole as it was required from the dramatic reveal of the four-toed statue. Would have been a bit boring to watch the camera to drift off 1000 miles in another direction!
Also the whole concept of an alternate reality (if that is indeed the case) seems like a cop out from the writers for me. It must have been difficult to decide which way to go from the incident, but you can't have your cake and eat it and do both. Hopefully this isn't the case and we won't end up with 2 endings which would be infuriating!
Apologies if any of that has been covered!