One thing that strikes me about the series is the number of boneheaded mistakes made by the castaways:
Not making weapons. Before the discovery of the US Marshal's weapon case (more on that in a bit), all they had were Locke's knives. Sayid should have applied his mechanical aptitude toward fashioning crossbows using metal and cable from the plane wreckage. You never know what's in that jungle - boars and "Others" and bears, oh my!
People walking in the jungle alone and unarmed. Enough said.
Not constructing durable shelter. Come on, over 40 days on the island and people are still living under tarps? You'd think Sawyer of all people would be building walls after that boar barged into his campsite.
Not addressing the language barrier. Apparently nobody took the initiative to try to teach the Kwons some simple English words. Sun already learned English; due to the violent nature of Jin's employment with her father and its affect on him, she had planned on leaving him in Australia. She should have had the courage to confess this to him, telling him that she changed her mind and chose to stay with him instead of going to the car awaiting her at the airport, asking for his forgiveness, and insisting that he start learning English so that he can better contribute to the rest and vice versa. It would have been preferable to the way that her secret came out.
Rousseau won't join the castaways. She can survive on her own, but if she ever needs a doctor she won't have the luxury of living next door to one. And she's holding back her own potential for contributing to who she should know by now to be her allies.
Failure to resolve the power struggle. Not counting Rousseau, the island has at least four super-sized egos: Locke, Jack, Sawyer, and "newcomer" Ana-Lucia. These egos have to somehow work together for the sake of survival. Locke vs. Jack is the biggest problem. There is no simple solution, but I'd like to see Sayid and Eko, both fairly level-headed guys, to work together discreetly to use their influence to ameliorate this ego conflict. Eko should be (and probably is) keeping a close eye on the volatile Ana-Lucia.
Locke and Jack vs. Sawyer is a piece of cake, if someone can get through the egos of the first two to recognize the key to dealing with the irascible con man. Sawyer doesn't respond to people pushing him around. He responds to respect. Hurley seems to get more cooperation out of him than anyone else, and he treats Sawyer more diplomatically than the rest of the bunch. Sawyer's better side comes out through his better moments with Kate, and even one instance involving Jack, when the doc trusted Sawyer with a pistol prior to the raft expedition. Sawyer's hoarding isn't good, but there's a way to deal with it. Make a deal - he keeps the stuff, Hurley is appointed Official Person Who Knows What Sawyer Has, and work out a way to trade for Sawyer's stuff when needs arise. Promise to build him some boar-proof walls or something.
Keeping secrets about discoveries on the island. Sawyer had scavenged items from the plane for himself, so no one really knows what all he has. Jack and Kate didn't tell anyone about the firearms that were in the US Marshal's case. Locke is the big offender here. He apparently saw the "monster" in the episode "Walkabout." He also found the Beechcraft and the hatch to the Dharma Institute installation. Locke and Eko apparently didn't tell anyone about the spliced-out orientation film. Michael didn't tell anyone about the text messages coming through the computer. The existence of Henry Gale is currently a "state secret," known only to Jack, Locke, Sayid, and Eko.
It is important that all survivors know what resources and what dangers exist on the island. Maybe there's items in Sawyer's stash that could be put to practical use Jungle travels could have been made safer with armed escort (which wasn't a standard practice even after the gun stash's existence was widely known). Locke's encounter with the "monster" revealed how to survive it - just stand still calmly; you'd think he would share this critical bit of info to the rest. Michael didn't know that some unspecified disastrous incident happened as a result of someone using the computer for communication purposes. If Gale really is a rich CEO guy, maybe someone at the camp saw him in the news - or maybe his social circles overlap those of rich CEO guy Hurley. Discovery of the hatch could have been followed with an all-out search for another way in by the whole community. The handling of the Beechcraft discovery doomed both Boone and Charlie. If more people had known about the plane, someone would have probably guessed the obvious conclusion of the laws of physics that A PLANE NESTLED IN A TREE CANOPY IS NOT SECURE ENOUGH FOR PEOPLE TO BE CRAWLING AROUND INSIDE. Perhaps a series of ropes could have been used to lower the plane to the ground, or at least to soften its crash landing. Also, Jack of all people would have secured the heroin out of others' reach. We don't know that Charlie is the only survivor who can be tempted by narcotics...
Some other thoughts:
Maybe Locke is wrong about the island - maybe it's a sadistic enemy instead of a friend. If some native entity is responsible for the renewed use of his legs, is that meant for good, or meant to get Locke into all sorts of trouble?
Maybe the numbers are actually lucky - creating "bad luck" out of "worse luck." For instance, the fire at the house that Hurley bought for his mom would have probably gotten a slower response if it hadn't been for the false arrest that placed cops right on the scene when it started. Rousseau is under the curse, too. If there is any silver lining regarding her situation, it is that her daughter Alex is not loyal to the Others who kidnapped her in infancy.
One theory on the drug used on Claire's baby and used by Desmond: it may be some psychoactive compound used to make the subject vulnerable to suggestion. It could explain why Claire was so trusting of Ethan and company (the medicine could have affected both mother and child), and how the now-deceased Kelvin managed to suck Desmond into the experiment.
Speaking of which, where is Kelvin's body?
Remember that cable Sayid found leading to Rousseau's camp? The other end of it went into the ocean. If I were Sayid, I'd be curious enough to find out where the underwater end of that cable leads.
Hurley was right; Sun Kwon is hot. (See memorable quotes under the Lost IMDb listing.)
Update: The Lost fansite Fuselage.com has a pretty cool icon gallery. Some of my favorites are: