[b]Lack of communication.[/b] It's important to form good relationships with the people you're roleplaying with. It's easier to keep the RP alive when you're friends with them. So communication is important, and conversations don't always have to be about the RP itself. Get to know eachother. IC and OOC.

[b]Lack of commitment.[/b] Another major thing that contributes to the death of roleplays is the lack of commitment. We all lose motivation sometimes, but if we're commited, we won't quit despite the lack of motivation. At the very beginning, you have to state what you're expecting from the players. Let them know that you're looking for commited players only, and if they can't commit, they shouldn't join.

[b]Different schedules and pacing.[/b] Life gets in the way all the time, so establish sets of rules when it comes to posting. Find a certain pace which everyone can agree to. There are players who want to post every few days, but there are players who can wait 3 weeks for a post. It doesn't matter, as long as you all have an agreement. 

[b]Bad writers.[/b] They ruin the experience for other players. Players don't have to be super good at writing, as long as they know the basics. (Grammar, spelling, punctuations, etc.) Always have standards. Let the players know what you're looking for at the beginning, don't criticize their writing when you've already accepted them into the roleplay. 

[b]Underdeveloped ideas. [/b] Flesh out your ideas before posting them. Sometimes, we get a good idea and we want to turn it into an RP immediately, but don't. You need to plan it out first. Flesh out the plot, settings, etc. You need to figure out how you want the RP to progress. You can't just think of a "starter" and an "end goal", you need to plan out how you'll get to the end goal while keeping the story fun and interesting. Always have a plan. Some GM's think that a "premise" and "goal" will be enough to keep the rp running. Rarely happens.  

[b]Boring characters.[/b] GM's shouldn't accept applications just because they feel bad. They should pick interesting characters which will fit in well with the story. 

[b]Poor GMing skills.[/b] GMs must always notify their players and communicate with them. They must have everything planned out, etc. I don't think I need to explain this further, we all know what bad GMs are like. 

[b]Dwindling interests.[/b] But this wouldn't happen if we all considered everything that was written above.