After a long run of straight DMing, I've switched to DMing one game while playing in another (same group, alternating weeks). My eyes have officially been opened.
Not that I was bad before. I was open to questioning (as opposed to direct challenges). Direct challenges made me defensive if delivered in an adversarial way, but questions are fine.
Example:
Me: The creature avoids all but a little graze of your greatsword.
Them: That's stupid! You're letting it live because you want to do more plot stuff!
(bad)
Me: The creature avoids all but a little graze of your greatsword.
Them: Okay, I've swung a greatsword a lot of times in my career as a paladin. Did this feel like he dodged most of it with his skill, like he's got a lot of hit points? Or did this feel like I should have hit him harder, but something made it not hurt as much.
Me: Good question. You really thought you had him, and you're not sure why your blade slowed down as much as it did right before hitting him.
(good -- I neglected to mention the fact that the creature had DR, and the player made an excellent case for his character knowing about DR)
As a DM, I've gotten a
lot better at describing things to my players after playing and being penalized for not asking to look at things I thought I didn't have to look at. We lost treasure for burning the body of a wererat. I didn't think that my character would have just burned the body if it had pouches and bags on it, but I wasn't told, and I didn't know to ask. Now, as a DM, I try to be very clear about what people see, and am very forgiving about the "Oh, if I'd known that he was glowing bright red, I wouldn't have charged him" thing.
With regards to rules stuff, we've played long enough now to have a pretty good grasp of how things work. I forestall most problems on tricky things by having other people do the work for me.
Wizard: I cast my spell.
Me: The monster uses its readied action to move into a grapple.
Wizard: D'oh.
Me: It succeeds on its hit, and then, with its grap... wait, you've got Deflection. Would deflection help your grapple check? Rogue, you're not up for awhile. Read up on it for us, would you?
Wizard: Can I draw my wand and use it without penalty in the grapple?
Me: Got me. Check up on it. We'll assume you're grappling at least for the rest of this turn. Meantime, it's Fighter-dude's turn...
The players don't argue me with me, now, because they're checking up on stuff for each other -- people who are waiting for their next turn volunteer to look up encumbrance rates while the person whose turn it is goes through his strategy. We all know enough about the rules to be generally cool, and I've built up enough DM cred that if I do something weird, I get a question to clarify, not a complaint:
Them: "Okay, so, normally in this world, Dispel Magic works like it does in the book, right? d20 + level against 11 plus caster's level? So if this is something different, do I get any feeling for that?"
Me: "Sure. You get the sense that this magic might have some odd feeling about it that's making it harder to Dispel." (Translation: Yes, I know, I'm having it not work the normal way, and I'd like you to trust that I'm not just screwing you over.)
Them: "Right. I'd like to head back to the town and look up instances of magic that is buffered against normal Dispels. Maybe the cleric could look up curses and see if there's something special there?"
And so forth.
Collaboration, cooperation, trust. You get that, everything else is just logistics.