Have you noticed this? At least for me, whenever I think about replaying any Monkey Island games, I always am mostly excited about that first chapter or exposition of the game. For some reason in all of them it seems like that's where the most magic and fun lies, and then it kinda drops off a little as it gets more towards the end. It usually goes into slightly weaker, less atmospheric territory, and more linear gameplay, almost like all the great puzzles and fun scenes were kinda expended on the first part and the end is more rushed.
Arriving onto Melee Island and completing the trials is what I really replay MI1 for, to me it's much more fun than Monkey Island itself. Getting rid of Largo and getting the 4 map pieces is the real joy of MI2, as well. LeChuck's fortress, Dinky Island and the last scene is less open and involved, and really not what I replay the game for (though the story is still great). The carnival in MI3 feels the same way, at that point i feel like all the cool scenes and puzzles are behind me, and the end scenes are kinda lame. MI4 definitely starts dropping off once you finish the first two islands, and the monkey kombat stuff is like a chore.
Is it just me, or is this kind of a trend for the MI games? When I replay them, sometimes I almost lose interest once I'm done with the first main portion and then know what's coming next. It's like I got what I came for, and if I finish it, it's mainly for the sake of completion.