Apparently Anna Lucia wasn't liked enough to bring back in, plus she was busy with some other projects. And They gave us that garbage explanation for Michael earlier in the season which I find ridiculous considering Sayid and Ben killed way more people in much more malicious manors. Evidently his soul was un-redeemable to lost writers?
My assessment of the finale having been an avid watcher and film school graduate is the following: The writers chose to have 2 major story arc concerns, 1) Character/Personality Development and the other 2) Mythological & Science vs. Faith storylines.
If you were a fan of Lost for the Character's, and interpersonal struggles and relationships, you probably were pretty satisfied with how things came to an end. However, if you were more a fan of the Mythology/Science of the show, you got pissed on as far as answers. Like , did we ever find out WHY Walt was so "special"? That was wrong of them to drag people along for 6 seasons and then NOT give several answers to questions they posed, no matter how trivial. My opinion, I could go on about this, but I'll stop there.
P.S How they did Micheal was supposedly planned from the beginning since the writers claim that they knew from the beginning of the series how it would end. JJ. Abrams & Darlton said they wrote it and locked it up for the 6 years, as not to be influced by fan popularity. Still a bunch of B.S if you ask me.