Going all the way back to the OP - Ydars - what made you learn lessons as a player this time around as opposed to all of the other times? Just curious.
For myself, I've learned the most about DM from being a player.
Many people have said this and even the OP, but I'll reiterate it for emphasis - nerfing character abilities sucks. Telling players, "Well, that Feat doesn't work like that." Or, "that spell doesn't work like that" is pretty crappy.
I especially dislike the nerfing of abilities for the sake of plot. For example, let's say we are investigating something. My survival skill is +17 and I try to Track where the killer's gone off to. Then, the DM tells me, no matter how well I roll that the tracks lead off in X direction, but then disappear.
What? So my Tracking ability told me...nothing? Let least give me a bone. Tell me how tall the killer was, whether it was a man or woman, perhaps if they were human, elf, or dwarf. But so many times, I've seen DMs completely nerf "plot breaking" abilities for the sake of their story.
When I run a game, if you've hit a high DC, I will try to at least give you something. Some kind of information.
Another thing that is related to this is loosey-goosey dice rolling. I personally hate that. When a DM tells you to make a roll. And you do so. If you happen to roll "high" on the d20, you succeed. If you happen to roll "low" on the d20, you fail - even if your skill makes a low roll high.
For example, if I try to track and roll a 6 on the d20 roll, but have +21 to my roll, it's still a decent roll. But I know so many DMs who might tell me - "Yeah, so you fail." "But I have a +21 to track stuff. That gives me a 27." "Yeah, you barely missed it."
The reason I hate this sort of thing is that when a DM does this, it makes me feel like I put all of those points and character development into my skill for nothing.
So, to combat this in my own games, I try to give my players the DC before they roll. This way, they know that know I'm not BS'ing them and it keeps me honest.
But the #1 thing that being a player cured me of is "pure whim storytelling". I know that many people love this. And, in fact, I used to be that kind of GM. However, when I'm sitting across the table from a GM who's making it up all on the fly, I can tell. And it seriously hurts my enjoyment of the game. Suddenly, I can see all the strings on the puppets. I can see all of the camera crews and special effects guys. Also, GMs who do it all 100% on the fly tend to be pretty loosey-goosey with their dice rolls. Often times, if what you're attempting sounds cool to the GM, he'll let you do it. If it doesn't sound cool to him, then he won't let you do it.