I was inspired to do this post after reading this thread here. It was a discussion of DMs who were talking and acknowledging whether they are a 'bad' DM or 'poor' at DMing.
So after some perplexed self-reflection, here are some of my own DMing weaknesses that I will admit to a fault.
Unable to run a game on the fly
I think a really great game is one when the DM is prepared, or at least gives his players the impression that he is in control. As a DM, I plan my scenarios as extensive as possible, trying to come up with any possible solutions that my player can think of and try to lead them towards a certain direction of where they should go.
Sometimes I get overconfident with my prep that I give my players endless choices as if they are in a sandbox environment and would be able to adapt to any of their actions.
Still, players will always be players. They can always think up of something that will throw me off my railings and now I have to find a new way of approaching the scenario if they have not avoided it altogether.
When that happens, all prep has been thrown out of the window and it's time to improvise which leads me to my first weakness; I'm bad at improvising.
When all my plans have gone down the drain, I have trouble coming up with something original at a moment's notice. I can come up with different new ideas for a game but it takes time and mostly involves alot of time sulking in a corner or hiding under my basement, doing just that.
My only way to keep the game moving is to rely on stock, stereotype and typical NPCs (like the dwarven blacksmith, the fat, gruffy bearded innkeeper etc.), plots (saving the damsel in distress, killing dragons for XP etc.) and monster encounters (goblins, kobolds, orcs etc.) which ultimately leads up to the point of achieving nothing except telling my players that it is 'The End' (XP & loot) for that session. That's if they don't get too bored with the session at mid-point.
My game relies heavily on good prep and getting organized before the game is pretty important to me because I must be able to sort out what happens when things get rolling. When things get out of hand, all hell breaks loose and what would have been a great game becomes dull and boring.
As I've learned, my only guideline of winging it is this; Whatever you do, just make sure it is going to be fun for the players. Even if the plot arc is on its way to ruin, just bury it up with some whacky and crazy encounter and end the session after that. Your players will still come out of the game having satisfied and having a good time. If you can't save the plot, save the fun.
I don't like to tell my players that they've de-railed themselves from what I've prepared for them because that usually means the session has to end there and then (but I'll probably tell them after the session). My players respect me by giving me time to prepare for the next session and I also respect them not to say that their choice of action would ruin the game and therefore should not be allowed to be taken. So by admiting that it is my fault for not preparing ahead in the middle of the game is like throwing a wet blanket over them when they are suppose to have fun.
This is quite related with my first weakness. My combat routine does fall into the 'you hit, you miss,'. I've tried to come up with descriptions during combat, but it also comes down to 'you swing your axe and you hit'.
Also combat as draggy as it was in 3.x, reduces my energy level somewhat. When an encounter runs for hours, the tension and pressure dies down and all we ever do for that afternoon or evening is taking our sweet time to roll dice until someone dies then we feel good about it and move on. However, by then the momentum of the game and myself have long grinded into a halt which shows for the rest of the session. I personally like to blame the system for that but it is also part of my job to keep combat exciting, so I'll admit that part of the fault is mine.
These are my major weaknesses which I believe can lead to or cause minor weaknesses that I'm not aware of at the moment. The best way to solve your problems is obviously talking to your players. There are the ones whose feedback you should listen to and only they know how you can improve on them. Afterall, RPGs is about cooperative gaming right?
So, do you have any DMing weakness?