A lot of valid points have been covered, here's some extra. 

I'll mostly be dealing with technical matters. For the creative component (ie. story writing), remember that open world is merely an illusion. Unless there are specific mechanics supporting contradictory posts, your RP will go down linearly.

When in doubt of your creative abilities, consult successful RPs. Skim finished games, ask reputable GMs for advice; there is no shame standing on the shoulders of giants. 

[b]■What are great ways to keep players engaged both IC and OOC?[/b]

[indent]As much as the GM gets players to engage, the players have to stay engaged themselves. If they keep writing low effort posts and not actively participating, perhaps its better they don't participate at all.[/indent]

[b]■Should the character submission be the only factor in accepting players?[/b]

[indent]Absolutely not. Being a posting member is a duty, not a right. A GM should involve personally during the application process. That is, talk to the player, see the way they interact. In addition, let the candidates mingle in OOC before launching. That way, a basic filter is applied to the good, bad and ugly.

Evaluating the players should be on a continuous basis, which means, once they're in, they'll have to prove they belong in. When someone's quality (IC and/or OOC) drops, the GM should consider dropping the said player as well.[/indent]

[b]■How do you deal with the players who have seemingly disappeared from the RP?[/b] 

[indent]Safe to say, the wise users above me have provided various ideas. The better question is, would you let them back in?[/indent]

[b]■In the same vein, how do you effectively prevent the above from hindering your other players?[/b]

[indent]As a GM, the game should be robust at its design. Meaning, the progression must not be hindered by individuals. For example, a game could rely on replacements. Keeping the interest check bumped and inviting potential new players at intervals. Making player characters expendable helps as well. I know some RP depend on every character filling vital role(s). But even for those, consider that is it really necessary to have the same guys the entire time?

Whatever strategies the GM employs to deal with hindering players, they have to be prepared in the first place. People will lag, drop out and act (willingly or unwillingly) against the greater interest.[/indent]

[b]■What is the most graceful way to end your RP due to inactivity or (*gasp!*) your own loss of interest/time?[/b]

[indent]Appoint a co-GM, set a line of succession. I've seen threads abandoned by the OP resurrected later, solely on the wills of the players. Robust systems have redundancy; even if the GM never imagine quitting out, appointing the next-in-line won't hurt. True mishaps are always unexpected, and you'll never know when your co-GM catches you fall.[/indent]

Edit: The dedicated GM stays loyal to his/her thread(s), and avoid biting off more than they can chew. Impatient folks start new RPs every time a light bulb goes off in their heads. This is creating fillers, the instant gratification unsustainable for the long term. Instead funneling the ideas into a new game, consider adapting them to existing ones.