I'm in the same camp I think a good RP is equal parts responsibility of the DM for making an interesting world/topic in the first place and keeping people invested, and its the responsibility of the player to make posts that are interesting in their own right, fun to read, help progress the story the DM has set, and above all, be active. I've seen (even been a part of) many RPs that, by using these elements, have managed to keep running for YEARS.
Again though, I do enjoy a challenge, so for me (and most people who I've rped with) the challenge itself is a large part of our interest- it is this constriction the DM places on us that tells our writer within to wake up and say "Put a block on me, I'll smash it. Put chains on me, I'll tear them out."
Again, the part of the DM lies not in just posting an intro and telling players what to do/not do. The DM must get involved and active about it, and put forth an interesting topic for people to show interest in. I mean, if you can't be passionate about your own work, how can you expect other people to be passionate about it?