Yay, Sayid! Unlike most people on the internet, I never stopped liking Sayid because he shot young Ben. I always took that situation to be the classic "would you kill Hitler as a kid" question, so I can sympathise with Sayid's situation. Honestly, Roger really gave Sayid what he deserved, so I didn't have any Sayid-hate in Season 5. If he stayed dead in the Temple, or died during The Incident, I would have been happy. Where Sayid went off the rails for me was after he came back from the dead... yet the actor played the character like a damn zombie. Detached immoral robot = boring, so I was pretty happy when Desmond pierced through his shell and gave Sayid's humanity a chance to assert itself. Considering everything he's done, I though his self sacrifice was a great way to end the character. Thumbs up.
Sadly, Frank's death was about as long as his lines in each episode. If you weren't watching closely, you missed it entirely. I liked him in Season 4, but he sure got the short end of the acting stick for Seasons 5 & 6. Then again, you can say the same about Miles in Season 6...
Jin who? Jin hasn't been an exciting character since... ever really. He was interesting in the early seasons when you saw the raw emotions of Jin and Sun's relationship (building, breaking and building back up on the island). I would have been fine with him dying on the frieghter at the end of Season 4, mainly because Season 5 & 6 consisted of nothing but him saying "I won't leave if there is a chance to find Sun!". Flat character, but he had his good moments and a nice death scene.
The death of Sun is a damn tragedy, IMO. I loved the transformation to the timid/fearful wife of season one to the bad ass woman who takes charge of her destiny. Her transformation began when she stood up and faced down Jin early on, with her confidence building four seasons which peaked at her facing down her own father and Charles Widmore himself. At that point, I was very happy with her character, and was hoping that the death of Jin on the frieghter would make her even more driven and determined. I was happy to see her with a gun ready to kill Ben at the start of Season 5, but unfortunately Ben pulled out Jin's ring and Sun's character unravelled and never recovered. Ever since that moment, she basically did whatever the rest of the group said, punctuating her speech with Jin-isms. "I have to find Jin!" blah blah blah. Four seasons of building strength washed away with two seasons of weakness and confusion. If you needed to manipulate Sun (which even Ilana did), all you had to say is that you knew the way to Jin. That's a damn shame for that character to be wasted.
At least their final scene in the sinking submarine was excellent and was a much better emotional reunion than what we saw last episode. IMO, their reunion wasn't even as dramatic as when she was first reunited with Jin back in Season 2 (the same scene when Bernard and Rose were reunited). Perhaps if the electric fence zapped them there would have been more electricity in that scene. ;)
As for Ji Yeon, she joins Aaron and Walt with characters with parental abandonment issues. Hopefully these three are addressed before the shows in, but I'm not holding my breath.