I was pretty disappointed in the finale, too. I actually think the last two seasons have been big improvements for the show. The first season was great, but the second and third had all the signs of them flailing around trying to figure out what they were doing, with characters running in circles and plot points sometimes contradicting themselves in the same episode (the second season finale, where Locke tells Desmond that the button does nothing, Desmond is all "Oh, I hadn't thought of that, I guess you could be right, brutha!" and then Desmond remembers 15 minutes later "Oh, wait, there was that time I didn't push the button and a giant magnetic explosion almost killed me and destroyed the world. Slipped me mind!")
But TE's right, the casting has been a huge part of turning it around. They struggled with some additions in 2 and 3, but all of the people who showed up in season 4 on - except 2007 Hawking - have been great. I loved everyone they brought in from the freighter, especially Miles and Lapidus. Jeff Fahey is bringing it whenever they put him on camera.
Still, the finale was a lot of what they do that bugs me. Characters keep making their trademark changes of heart - Faraday just revised his time travel theory because the season was ending, Juliet turns into an emotional loser in the finale as Jon says above. And all the black/white foreshadowing stuff aside, saving Jacob's evil other until the fifth season finale and not even naming him until the final season feels kind of cheap.
But Locke's corpse tumbling out of that box was a pretty terrific "!!!" moment, and I do want to go back now and watch some of the episodes from this season to see how much they were tipping their hands about Black Lodge Locke after they get back to the island. They've given Terry O'Quinn some rough patches to work through with Locke, but the guy is consistently great.
And really, I'll be happy next season if Sayid survives the gutshot and atomic bomb, because I don't believe either is enough to kill him, and he spends next season killing Jacob, Evil Jacob, Ben, Widmore, Faraday's mother and the magic smoke monster by himself.