There's a balance you need to maintain when considering what you label as 'serious' roleplayer. Having played across servers with compete freeform and no rules, and servers with long application processes, I found two extremes in NWN RP and the pros and cons to each:
The first is on the side away from strict, serious RPer. Total freeform, no rules, no restrictions. The elf-dragon mentioned? Oh yeah. They're here in the bar with us. So are the surface drow, the teiflings, the celestials, the half dragons, the strange half-canine fellow over there by the jukebox. Storylines don't know limit. No brown and drab 'realism' to get us down here. Fantastical is the norm.
Pros: Complete creativity abound. I have played on servers with this extreme and absolutely loved being able to craft whatever history or story caught my interest that day. Maybe I feel like hunting in the woods as a satyr in the morning, and after lunch stomping around as a slaad, I can do that. No one cares, they're all here to have fun. Fun is the name of the game. No one is afraid to propose radical and new ideas. This can lead to amazing stories and their seriousness is entirely up to the players involved.
Cons: The environment can attract and breed the 'my PC is better than yours' one-upping. Because fantastical is the norm, to be unique or different, you have to go to extreme lengths to do something out of the ordinary. There's no big reveal generally and drama has to be pumped to insane levels just to mean anything.
The other side of the spectrum is the deep, serious rulesy application-only roleplay worlds. When you get into them you know you're in with people vetted by a process to eliminate anyone who doesn't want to play within the pre-set lore of the sandbox. You're not going to run into random elf-dragons.
Pros: If you're getting into a server like this, generally you're in for the long haul. You find people with long, amazing, deep stories and three dimensional characters. Character development is a big deal, and you can really connect with the story. If something fantastical really comes along, it means something big.
Cons: Such structures and rules, especially with an application process, also tend to breed elitism. It's not a healthy community feature to collectively say 'we're SO much better than server X because we're much better roleplayers.' Egos can get puffed, and the us-VS-them will eventually turn inward on the community itself. Because you already had to prove yourself to get in, you may not be brave enough to strike out and try something radical and new, sticking to the same RP that gets you the praise of your peers.
The best servers I've played on have always struck a balance somewhere between those extremes, but the biggest contributing factor to maintaining that balance and the atmosphere is how you present your server at the first look- your forums.
I always like to read back on forums when I check out a new server. If I see public drama or drama spanning multiple threads, I'm less likely to give the world a test run. If I see that the rules are 20 pages long, or linked across 10 threads with obscure, hard to find tidbits, I'll look elsewhere. If I see a public wall of shame, I'll start running for the door.
If I see general forum banter, screenshots, themesong threads, character bios, interesting discussions, then the playerbase looks like a nice, sane group of people. This is where you can hook people- even if they do run into Mr. Dragon-elf PC. If they see 30 character bios of 'normal' (for the server) PCs, then the Dragon-Elf's power to represent your world diminishes.
That was way more longwinded than I meant. Haha. I'll get to my point.
You need to decide first how you define 'serious' roleplayers, You need to know exactly what you want to attract. When you've got them, nurture them. Make your world thrive on the players you want, and the'll multiply like bunnies in the spring.
