(Just noticed that Paludosa2 even points out how the film kind of recycles character dynamics as well which is as distracting as the visual stuff I mentioned.)
It's a beautifully animated film but for me it's one of his least interesting films. And something about the characters just never clicked for me, I was ok with watching them but I never really cared about them. Everything felt like something i'd seen before which I never feel when watching a Miyazaki film. It seems like a nit picky thing but I can't stress enough how much it took away from certain moments or story beats. Not only was I not really having an emotional connection to any of the characters but the insane imagination that usually makes his films so magical felt like it wasn't there.
When first watching it I couldn't get past the feeling that I had seen this before. Usually one or two of these will show up in his films in some way amongst a host of new ideas, but they all show up in Howl's. The visual things include flight, airships, steampunkery, anthropomorphised objects and black globular creatures, among other stuff. The main thematic ideas that show up a lot are unjust wars, environmentalism, young girls in peril and more I can't remember. I watched it at the end of a week where I watched all of Miyazaki's films and couldn't help notice that Howl's contains almost all of the imagery and themes that Miyazaki likes to dip into. Out of all of Miyazaki's films that I've seen, everything bar Castle of Cagliostro, Howl's Moving Castle may be my least favourite. And, remember, these are also children's movies- try and watch them without our adult disillusionment. I don't think it's as great as Spirited Away, but at the same time they are VERY different films. Yes, it harps on Miyazaki's favorite themes of war and environmentalism, but I still love watching it. I love watching this movie and often recommend it to my friends. I also love how much character the "castle" itself had, like a living beast. Howl was a MUCH bigger jerk in that he projected his whiny selfishness more than in the movie (where he does do it, but in smaller doses). For example, the Witch of the Waste was quite beautiful, and I think could change her appearance (if I remember rightly). It's been a while since I've read it, but some differences are still noteable in my mind. It is, in fact, quite different from the book, almost a totally different animal. I absolutely love the world he creates, the whimsy, the gorgeous animation, I totally identify with and love Sophie, and all of the minor characters are adorable (Calcifer, the dog, etc). I agree with many of these posts that it isn't particularly "different" or an upward trajectory in quality for Miyazaki from his previous films, but that doesn't make it any less enjoyable for me. I saw it in theaters when I was pretty young and had no idea what I was in for, my mother decided to take us and didn't really tell my brother or me what it was about, hah.