Tons of little things. Hard time remembering specifics now but...
The "Across the Sea" episode was just disappointing all around. All that build up and we get bad child actors talking vaguely about nothing and have an episode which really didn't give us anything.
Especially when "the rules" don't remain consistent unless it's plot convenient.
Ben can't kill Widmore, it's against the rules. Oh wait. Bang.
Desmond's dream of Claire getting on a helicopter, though that's more of a nitpick.
Then there's all the things that were just left hanging: timetraveling outrigger shootout, Ben's female friend (Annie?), Walt (I get that his actor grew up. They apparently decided to make him pointless. But despite being pointless, he wasn't written out, showing up a few more times both on and off island). Those don't bother me as much, though I do feel it detracts a bit.
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##Alec