One reason for my earlier flippant reply was because I wanted to offer advice as somebody who has been GMing for almost 28 years but I find that much of it is not applicable to a
PbP game. Things like pacing or ignoring rules for a quick resolution in the middle of a session are meaningless in that context. So there are people much better qualified to offer advice for that medium.
However, from observation, running good combats is not the principal skill required for a good
PbP game. It seems to me that they are much more reliant and focused (in general) on a strong setting that hooks in good players and an intricate plot, laden with hooks, to keep the players coming back for more.
Honestly I don't much care for
PbP games for myself. I feel the need for some level of real-time interactivity with the participants in the game, even if we're not face to face with each other. But I've played one before and read along with a couple more as they progressed. The ones that seem to work best start with and maintain a fast pace of posting over an extended period of time.
That is difficult to do with a combat focused game because inevitably you have somebody who can't post as frequently as the others do at a given time. And the creates a problem because, unless you plan to "skip" them when it is their turn in the battle, you can only move as fast as your slowest person.
Non-combat situations seem far more forgiving in that respect. The PC's can roleplay off of each other and make a few skill checks without everybody needing to participate at the same level. To facilitate this you need either a really good plot with lots of interesting aspects to it or really good players who will take any shred of plot and run with it. You aren't guaranteed the latter so its better to make sure you have as much of the former as possible.