The reason I decided to just "like" Under the Dome, instead of "really liking it", or even "loving it", is because King pulled what I like to call a "George R. R. Martin": giving the story too many characters, many of whom die, most of them unimportant.
That isn't to say the story doesn't deserve to be liked. It does. I just can't stand reading about certain characters that seemingly have no purpose.
That being said, the main characters you do get to know throughout the story are well rounded and have good development. You do care about the protagonist, even if you don't care about the 30 other side stories trying to steal his spotlight. The antagonist is justifiably hated. The relationship between the two is tense. It's good. It's just not great.
I'm really hoping Part 2 offers up an interesting and unexpected climax. I'm hoping King doesn't decide to pull a "George Lucas"; "aliens did it" is not a satisfying ending to a 1200 page book. Here's hoping ...