You know, if they had brought back the original cast in their original capacity, I wouldn't have minded the dramatic shift in character focus. We already know all about the characters from the first film, so the film's rush into its action (Godzilla appears in the first 30 minutes and makes landfall shortly after and the movie has another hour to go) wouldn't bother me as much.
I'm also not fond of the whole, "Mothra will help you, but only if you don't use Mechagodzilla." [Mechagodzilla appears] "Eh...f*ck it, Mothra will still help you" thing. Why make a point of it if NOTHING comes of it? I mean, I know some people complained about the DNA computers and lack of re-emergence of the "Mechagodzilla running wild" thing in the first film, a complaint that I don't really share, considering it's often brought up that Kiryu may run wild again and everyone always seems pretty nervous about it....but in this case, a plot point is legitimately brought up that serves NO PURPOSE AT ALL.
I like this movie a little more with every viewing and I'm quite sure that as a kid, I'd have loved it with all the monster action beginning so early, but it's got some problems and doesn't work quite as much as its predecessor.