You want to know what makes a good plot?
Logical, believable conflict.
Explanation: A has [dispute] with B. A desires [object/entity/concept/etc (C)], but B's perspective is in opposition to A and would deny A [object/entity/concept/etc (C)]. Therefore, A and B are now in conflict over C. C must be valuable enough to justify the level of conflict involved between A and B. Notable, this can be changed by A and B as well. For example: It wouldn't be very hard to believe that an obsessive, greedy, lecherous madman (A) would easily fight to the death with B (heroes/faction/etc) over C (damsel, magical macguffin, etc).
There. You're done. That's a good plot.
No, seriously, look no further.
Good, solid, stable, simple plot + good, flowing, interesting characters = winning combination. Examples range from Castle to Babylon 5. In fact, most successful RP's on this site ironically end up using this simplistic formula for it.
Hell, here's one.
Romeo & Juliet (A) want to get married and live happily ever after together (C) but they cannot due to their families (B) who are in constant, deadly feuding (conflict).
Done. I just took Romeo & Juliet's premise and plot and put it in one line. *Shrugs*
You can add complexity, and twist the formula however you like to fit your own desires and tastes. Like maybe you want A and B to be in high tension to create drama, but they're both attempting to avoid/survive/defeat C, perhaps by accomplishing D. There's an example of twisting the base formula.
However, in the end, no matter how you twist the formula, you need a conflict, and you need it to be logical and believable.
Hope that helps.
Simply put: Most plots in RP's here fail horrible because the conflict is lost. Or the conflict is there, but it's not remotely logical or believable. When something is boring, it's not fun to play. When something breaks suspension of disbelief, it's hard to take seriously. A conflict must be present in the plot for the plot to have point and purpose. On top of this, at least specifically for RPing, the conflict must be shaped and changed and affected directly by the players. If you have a conflict going on in the background and don't let the players resolve it, you're hand waving the purpose to having players.
If you have more specific questions feel free to ask.