Watching it now, I'm not entirely sure why it is a really liked it as a kid. It could be because we got to see Lugia and the bird trio, though it could be because we actually got to see Ash save the world (again), and that was rather intense for a movie, even if the first one is pretty dark itself.
I don't know anymore. As a kid, you don't think about these things much, so when it finally hits you when you're older, you start to question why you liked it so much.
I don't really like this movie, but I'll watch it if I'm bored. The premise sounds nice on its own, but I really think they were pushing it a bit too much here. This really was the first instance of Ash being the chosen one (though I think that was more 4KIDS' doing), but after this, we never saw anything else that points to him as a chosen one until Sinnoh. I guess they figured they should give us clues about Ash's potential as a character, and decided to put it to the test here. But it really wasn't that well executed. Legends written thousands of years ago like that seem to always come true whenever Ash is around, and I find that silly. Sure, I can let it slide if it happened recently, but this was when he was in the Orange Islands. How were we supposed to know about Aura and all that?
I honestly don't know what goes through the writers' heads when they come up with this. I think they're just trying too hard to link movies and the episodes together like that.
I'm not a fan of the characters, to be honest with you. Melody seemed more to me like a rival for Misty than ever, and she was just there to look pretty and play the song. Ash is the chosen one, HE should be playing the flippin' song! Ah, whatever. Lawrence III has a rather weak reasoning for why he's a Pokémon Collector, but he does have a cool ship. Slowking was there mainly for comedic relief, I found no purpose for him being there. It's just... creepy, actually. They don't really question WHY he talks, except for one time, and even then it wasn't answered. They just brush it off. Lugia speaking made more sense since it was telepathy, but even then, it was kinda dumb.
Speaking of Lugia, the movie did capture the powers of a Legendary rather well. It showed that a Legendary can fight them off, but they do have moments of weakness, and so I suppose that was a plus. Though it makes me wonder if it was done on purpose, but meh. Still, he didn't do much despite being the title Legendary for the movie. Zapdos, Moltres, and Articuno did more than he did. He was there just to do fancy flips and look cool.
Team Rocket has to be my favorites here. I think this was the first instance we saw that is proof they aren't really bad guys. They can do the right thing if they wanted to. So the self-sacrifice they did was rather shocking at the time, and I really didn't want to see them go. But lo and behold, they live anyway, still debate on their life's work, and make a fourth wall joke at the end. I don't know why people think that's bad, I think it's actually kinda funny. It made more sense in the theaters, though, but it still hits home if you have other people around you watching it XD.
So yeah, not my favorite movie, but it helps kill time.