I think this board is mostly for actually coding support, but I think you have a bit about that up there, so maybe it's ok. Hehe.
First, making an awesome looking board:
A nice skin is a must. IF Skin Zone has nice ones. And you can always click the credit links at the bottom of forums to see where they got their skins from. =] For instance, RPG-D got their skin from Ink Drop Styles. Just take a look around and see what you like.
Organization is key. So you want to make sure that members can find important things (like the Rules, Character Bio Application, Plot, etc) easily. A good way to do that is with a
Side-bar. You may also want to make sure your boards are divided up neatly to avoid looking messy and clumped together.
Storylines:
Plots/Storylines are really up to you. It's probably best to have a set plot for all members to follow to avoid everyone getting bored. Storylines between only a few characters are usually called subplots. Subplots aren't really neccessary, but they can be fun to have if you want to see a specific story take place. Usually, subplots are set up with canon characters that members have to adopt to take part in the subplot.
For my RPG, we have neither a main plot nor a subplot. The members create their own stories, which I think is what a lot of people prefer to do. Just talk to your members to see what they want. It's all about making them happy. =]
Quality:
I'm there with ya, buddy. I've had the same problem. A lot of it has to do with where you advertise. From what I've seen, unless you advertise somewhere that experienced role-players gather, you're going to get newbies who don't know very much. I can promise you, however, that they get better. We all had to start somewhere; give them some time, and some guidance, and they'll improve.
But if you're looking for a quicker way to get high quality, I'd stick to advertising on RPG directories (like this) or on other RPG's with the kind of quality you like. Also, you may want to have an indepth character application. Having a bio that requires a member to really plan out and figure out their character will keep away a lot of the members that aren't good writers or roleplayers, leaving you the more serious ones. You can also try having a minimum post rule; such as, all in character posts must be at least 3 paragraphs long.