The Life and Death of Poor Writing
This was the episode. If you looked at all of the episode titles (like I did) before the season began your finger landed on this one and this was the one we were all waiting for. A mystery built on since the end of Season Three Finale and then continued in the Season Four Finale. The episode had high expectations and they were not met, at least not for me.
The beginning of the episode was very good. We get to meet Ceasar and Illana and we have a little bit of an extended scene before the show transfers into the flashback mode of the episode. My favorite moment of the episode was when Illana was talking to Locke and when asked what he remembered he answered "I remember dying." I do find it weird that Locke was pinpointed as "never seen on the plane." I thought that was a bit of a stretch. I'm positive that if I were on a plane I would never be able to tell you who was on it and who wasn't when I got off. I understood it in "The Other 48 Days" because that was right around the time they were trying to pin the traitor to the group and at least you had people frantically searching for a face on a plane and in order to agree they were like "Yea, I don't remember him being on the plane." It all worked out because he was on the plane - which actually kind of proves the point. This plane crashes and as soon as Locke wakes up he is questioned as to why he wasn't on the plane? I thought it was a really big stretch.
Into the flashback itself, it started off with great promise. Widemore being there when Locke woke up was a great scene. I also liked how Widemore knew what Locke was going through, even getting Locke to tell him that it had only been 4 days since he had seen him. Great stuff. Continue with Widemore, more great stuff. Widemore finding out that Locke left the island on his own free will basically gave Widemore the notion that he might be able to return to the island. I wouldn't be surprised if Ceasar and Illana end up being Widemore's people.
Matthew shows up, which was good to see. Locke then goes on a rough patch of people. The Hurley scene was not that good, the Sayid scene was very poor, and the Walt scene didn't even make any sense. He didn't even mention that he was Jeremey Bentham so I do wonder how Walt is able to make the connection when a man in Los Angeles dies under that name. Walt telling that he saw Locke in his dreams was the only real thing that saved these three scenes.
Skip ahead to Helen. First of all, love that he at least shows that he still cares about her by asking Matthew to hunt her down. A lot of people say that the gravesite was a fake, but personally I think they just needed Locke to see that there was nothing left for him in the real world. I do believe that she is dead and that we won't be seeing her character anymore. Matthew dying - not a big fan of this but with his commitment over at Fringe I guess they only had one real epsiode to use him. I loved the mystery surrounding his character in The Beginning of the End and the visit in Cabin Fever was amazing but I was not a fan of his role in this episode. I wanted him to have a much better, more important overall role in the show.
Locke and Jack. Worst scene of the entire episode. Correction, worst written scene of the entire episode. Terry and Fox did a great job acting in the scene, but this scene was so poorly written. Jack stated in s4 "He told me that a lot of bad things happened. He said it was my fault, for leaving."
Where? Where was any of this mentioned? This killed the episode for me and I was taken aback. Go back to your old scripts, look at them, and say what you were supposed to say. In fact, Jack said that Locke told him that Ben was off the island. He didn't even say that. Poor execution for the most hyped episode this season is going to have. I have a very strong feeling that this episode is going to fall on rewatch even more then it has all ready fallen for me.

Comments
well, the plane was intact, and if i remember correctly, they said he was in the water. which is DEFINITELY something i would question.
"Correction, worst written scene of the entire episode. "
There's no evidence that was their last interaction...i think you're jumping to conclusions too quickly here.
For Walt I agree, although I don't really see it as a big problem given how in the finale it was only a plot device so that the end reveal would be shocking. Although I still can say several ways of how Walt knew Locke's name was JB, but whatever.
You pointed out some minor things you didn't like... well, minor to me anyway, but you didn't even mention the Locke/Ben scene which alone is worth 9.0/10. As for Abbadon I think I disagree too, I doubt he got killed because of Fringe, he was just sort of a stepstone to Widmore, like Tom(he was only mysterious until the real leader was revealed) was to Ben... although this could be wrong, but I really liked his death here.
the episode was already longer than usual. They couldn't fit it in.
I think that the fact that the writers chose this really weird storytelling(we knew the main events of Locke's journey, the outcome of it, but we couldnt connect the dots fully) is the ultimate fault of the episode for the nay-sayers. They couldn't make it satisfying because people don't like recapping the same thing over and over again, but at the same time, it's nice to see it actually happening.
I felt that the info about Locke and the island was scattered throughout the meetings: he told Sayid his name, he briefly told people on the island, and told Hurley that they all need to go back. I think this is a result of last minute scripting/editing.