OK, We've had the good, now this is the bad (I still liked it though) :
1. Episodes where nothing happens in the plot and you get a boring flash back. The interminable episodes of Hurley's flashbacks (The DharmaVW van episode "Tricia Tanaka Is Dead") and virtually any episode with Charlie's band in the flashback. Fast forward material!
2. Jack's a bit boring. Maybe it's a flaw in the script, but his over-earnest acting started to get annoying by the third season. Particularly when compared to Sawyer; he never really changed throughout the series whereas Sawyer had definite character development.
3. Kate's a bit of a lightweight as well. OK, we get it, it's a love triangle! Just make your choice freckles, and get on with the plot ;-)
4. Locke vs Jack / Fate vs Free will / Man of science vs Man of Faith. Interesting at first, but became a bit tedious towards the end.
5. Llana, the french woman, Alex, Walt and all the other under-developed characters that the writers obviously planned to do something with but ran out of episodes to do it in so wrote them off! Naughty, lazy writing boys! ;-)
6. * New! Claire abandoning Aaron then disappearing and then reappearing as a mad women. I just never bought it. Also, Kate deciding to adopt Aaron for 3 years forming an emotional attachment and then abandoning him to go back to the island. Also, what happened to the two kids in the tail section? Kids in general were given a raw deal by the script writers