Smogon is heavily moderated, and thank the heavens for that. It is strict on the enthusiastic newbies who want to discuss their favourite aspects of Pokémon, and make wishlists and game ideas to their hearts' content. One liner posts are discouraged. In places, you have to ask the moderators to be allowed to make threads at all. It's not a very easy place to make yourself familiar with forum etiquette, you have to know the rules upon arrival, or you're thrown out.
Yes, moderating can be a killjoy, but the orderly structure of Smogon also allows for more intelligent discussion. The "elitist" attitude might discourage the glowing, enthusiastic, know-nothing-fans, but the ones who stay make pretty awesome discussions. Smogon is a place for serious discussion (apart from A Forum, which is in-jokes piled on in-jokes until only the mods know what is going on. Taking the threads at face value - like I try here - is futile most of the time. I don't even know why I post here, I should really know better). Even though the subject matter is quite childish on the surface, it's not (allowed to be) treated as such. We discuss the mechanics, not the interface, so to speak. Pokémon is a kids' game, that much is true, but it's also a pretty complex game when you look beyond the monsters themselves. We don't focus that much on the Pokémon the way you see them when playing the games or watching the anime, but the game they reside in and the ways to use them. The grown-up part, so to speak. Pokémon fans become fans because of the bells and whistles, but they come to Smogon to discuss the gears and pistons making up the battling aspect. Serebii embraces the "surface" a lot more, and thus draws in the crowd of "casual" Pokémon players. We do have a subforum for that as well, but you still have to follow the same standards that apply to the competitive parts.
My visits to the Serebii forums have been very brief, but it appears as if they are a lot more loosely moderated, making a more child-friendly atmosphere. In that sense, Smogon and Serebii fulfill different roles. Serebii introducing fans to Pokémon discussions, Smogon taking them to the next level. I'm glad both exist, and I'd be sad to see either of them go.