As a guy who has successfully failed to GM two forum games, I have a lot of advice to give you.
And here they are in no particular order
1. Don't let early success blind you to your original scope. Although this rule has several exceptions (Perpleixcon, Warrens, probably some other ones), suddenly deciding to put more effort than you originally planned to in your game is a good way to stretch it out to the point where it jumps the shark. I once decided to run a magic arena just to test a magic system I had for an RtD, and I managed to get several waitlisters. Overjoyed at my sudden success, I then decided to make the victory condition a bit too high, and so eventually as the game went on, I began to realize the flaws it had.
Mini-list:
1. Enviromental kills are a good way to kill a player without giving other players the satisfaction of killing someone.
2. They also aren't fun to die to since they tend to be just dice rolls in the grand scheme of things
3. My magic system was too shallow for the game I tried to run (fine enough for my own purposes, but not deep enough for an entire forum game)
4. I was too hesitant to kill my players (due to most prevented deaths being environmental ones)
5. I sucked at interpreting dice rolls
2.Try to make sure that you actually have a good ruleset. You know, fair, balanced, fun.
3. Design the game to your own strengths as a GM. If you suck at writing, avoid doing something narrative based and write updates simple and to the point. Players would appreciate frequent simple updates, over scarce, poorly written walls of text.
EDIT:
Here is a list of mistakes I've seen other DMs do.
1. Don't be afraid to fast-forward through some tedious and unexciting moments. A text book example would be walking down hallway after hallway of barely to completely featureless hallways. This is filller, it isn't fun! Unless these moments are short enough and you update frequently enough that it only takes a day to get past them, you should probably skip them entirely. But what if I need my characters to walk down this hallway? Then AUTO IT! Sure, you might piss off a few players who may have wanted to thoroughly search the room before leaving (or something else), but you are the DM, you know that room is empty! Also, your players would enjoy actions that had more impact.
And now for a less DnD example. Fighting a bullet sponge, If the players action over several turns boils down to "keep hitting it", then you've made a mistake. I will not propose a solution since I don't want to talk about matters I know little about.
2. Not enough guidance. Have you ever had those moments when a game slowed to a crawl? This may be why. Sometimes, players are overwhelmed by a new situation filled with way-too-many choices to make and so they make a bunch of meaningless ones that don't go anywhere. May sometime overlap with number 2.