Well, I'll go ahead and explain. To many fans (like me) this was the last true appearence of the original "people can just BURN!" Godzilla attitude. He has his own personality while being completelly, utterly ruthless. Mothra has never looks better, at least till GMM. The performances were very much above average, excellent for a Godzilla movie. The characters never wore out their welcome for me like they do in most G-movies (Case in point, I want to strangle half the characters of KKvsG and several of G:TTHM by the time 90 minutes are up). Godzilla also is in in best pre-90's suit by far, and the atomic blast effects are the best they have ever been in the original style (as in, not a giant cohesive beam like from the late '70s on). Mothra NEVER wears out her welcome, and her end is geniunely moving and beautifully filmed. This also has an excellent Ifukube score.
The most important thing Godzilla vs. Mothra did was that it DEFINED Godzilla movies. Before this, we had a great original, an entertaining but forgotten sequel, and a big revival/comedy/name-match brawl movie. This film was a GODZILLA movie. It defined the term. It brought back Godzilla's attitude as well as helping to make the Showa films one "universe" (bringing in Mothra) and establishing the overall look and feel of the Honda '60s Godzilla films. This movie is one of the reasons why I didn't dismiss Godzilla while I was maturing, and if I had I would not be talking to you right now and it's doubtful I'd have bothered seeing anything past Godzilla 2000. Perhaps most telling, it's a Godzilla movie normal people will watch! They'll say "That's the one with the little twins, right? I love that when I was a kid!" Then they'll watch it expecting a laughfest and get into it all over again. Even my dad will watch this with me if I corner him and beg (a distinction this shares with only the Gamera trilogy, GKOTM, and GMK).
In short, anybody who skips this movie because they don't like Mothra is doing themselves a disservice and anyone who doesn't recognize what it did for the series needs to watch it again. It defined the Godzilla movie and was the shining moment of the "Golden Age."
People certainly have the RIGHT to favor "Monster Zero" but I think they're nuts! The only efforts that came close in this era IMO were GTTHM and DAM, and even they got a bit too...silly. Damn if they weren't a hoot, but even if it isn't your favorite, you have to appreciate Godzilla vs. Mothra because it made those movies possible.
Because damn it all, Godzilla vs. Mothra was a GODZILLA movie and it wasn't afraid to show it and it looked good doing it! It was the pay-off of KKvsG's revitalization of the series. It showed that people were still interested in Godzilla. In the realms of realism, excitement, music, and characters, this film gets a 10 from me.