There are two things about this episode that can easily lead you in the wrong direction when evaluating it. First, it is a direct sequel to the monster that was "Hush", and having to be a follow up to something that strong cannot be easy. Second, it is about reopening the "Hellmouth" again. And that sounds repetitive. But what seems to be and what is may be two different things.
"Doomed" isn't a horror story even though it deals with another potential apocalypse. Unlike "Hush", it does not want fear to be above all else. Something different is going on now. Buffy, the titular heroine, has been through a lot in the show up to this point. And as much as she would like to, she no longer thinks she can have a normal human relationship with anyone at all. One reason for that is, of course, an experience that is rooted way back in the previous seasons. Not season, seasons, that is just how long that experience took. And another is rooted in this very season. They have made her very careful and defensive, more than necessary, possibly. Right now there is someone who could really be good for her, and they do feel something for one another. And it has been that way for a while now. But she has very little trust left. Then there is the new big threat to deal with immediately or it is the end of the world. What makes this end of the world stopping plot special, though, is a different take on ritual sacrifices. And then there is a character that has undergone significant changes recently, and it is interesting to watch Spike learn how to cope with his new self. In the end, however, "Doomed" wants the audience worried and concerned. But only to understand that trust is vital and everybody needs help and company. To stop the doom.