10-Coming up with this whole Jacob/light thing at the final season.
9-Making only a 5-day gap before one of the most anticipated episode of all time,they should've left the hype build on.
8-Raising too many mysteries than they could handle.
7-The love triangle thing is just too silly to be true.
6-The submarine submerge CGI in "Follow the leader" is very horrible and unforgivable.
5-Getting drunkies to perform key roles
4-Not striking a balance between the importance of every character(Jack has had over 30 centric episodes,while Characters like Claire Had very few)
3-Allowing a Lost game this awful to be put on the streets to haunt Lost fanatics in their dreams.
2-The Epilogue is meaningless,useless and Worthless.
1-"Stranger in a strange land",That was a mistake.