Because we have different sources for Shakespeare (Folios and cuescripts) there are some variations of text -- some characters have monologues that are either abbreviated or removed by the time the First Folio (FF) was published. My guess is that Shakespeare, like every artist, had bits of his work that weren't quite ready for primetime, or that he later chose to edit out. This may have created truncated characters in the text that we read today.
I particularly like the porter, and I find him the most humorous of the charactesr in Shakespeare's tragedies. That said, I also think that Fleance could have been elaborated on as he becomes Banquo's heir, as predicted by the witches.
The scene between Lady Macduff and her son always strikes me as extremely touching, and their deaths are among the most horrible in the play. The son's last words -- "He has kill'd me, mother: / Run away, I pray you!" [Dies] -- have stuck in my mind since I first read the play many years ago.
I always loved the conversation between Ross and Old Man. The fact that they noticed the parallels between nature going haywire and Macbeth becoming king was a wonderful one. I also believe that the Porter needs a few more lines. His humor becomes more and more hilarious for me each time I read the play.
I think that because the scene is a ghastly one, but it's also a heart-wrenching one, the gentlewoman who can't help pitying lady Macbeth observing her as a nurse her patient is interesting. Lady's Macbeth delirium is pain and rage shaped into beauty and the gentlewoman, in addition to being one of the "moral" voices in the play, is not insensitive to insanity combined with such mighty truth:"you have known what you should not."