The last time I GMed, my main problem was players who didn't want to play, or didn't want to play as a team. Intra-Party Co-operation was next to nil. Two PCs split (at different times), and the rest died.
As a player, I find the same problems. Something about individuals teaming up just doesn't seem to be working... Sure, there were some squabbles, back in the 1e days (like when the Paladin had more magic items than the Paladin's code allowed, and he wouldn't surrender the crossbow to the Monk, who had no magic items), but all-in-all, we were usually a team, and generally worked together. This never seems to be the case, anymore.
Another problem I've had as a player is a GM who, while great on world-creation, was awfully poor at description, but expected very detailed actions... I tell her my PC looks in the room, roll Spot, and tell her my total. "Looking for what?" is her response. Meanwhile, she tells us we see a fence around a parking lot (in a modern-day setting), and one of the PCs pulls his Leatherman (multitool) to cut the fence. No, she responds, it isn't a chainlink fence... Or the time she shows us a tiled map, saying we see a ruined keep. I ask about the condition of the walls, and she replies "Well, it's all pretty clear, if you just look at the map" (despite the fact that I had just asked a question about it). The Rogue later has to ask about the height of the remaining walls, etc.
A similar problem I run into, a lot, is GMs wanting detailed PC Backgrounds, before they tell you anything about their world. Then they inevitably tell you "No, that doesn't fit with my world! We're going to have to change this..."!
I still find the incompetence of PCs, at levels below 10th, to be pretty shocking. I want a game where the PCs begin as competent, and advance more slowly, I guess. I hate being told my PC can't do something I could accomplish in Jr. High. It destroys versimilitude, and any enjoyment of the game, for me. (DMs: Don't sweat the small stuff. If a seventh level Ranger says he pitches a tent, forget the Rope Use roll! Assume a Ranger know how to pitch a tent, move on, and get over it!)
The power divide between Spellslinger and Mundanes also bothers me. I need to write a lower-magic game where skills are as important as magic.
Example: In 1e, striking an Invisible opponent gave you a -4 penalty... Now, he is immune to being hit, AT ALL, except with area affect, unless you first locate him with a whopping penalty (-30, IIRC), or just swing at two random squares. Ees no fair! Spot checks which succeed (with an Invisibility bonus) should pin-point the Inviso, not just tell you "There's something out there, somewhere!" I have never figured out just WHAT is being spotted, if it's not tracks (he's in the last one!), something being moved (THERE he is!), etc.
Nope, spells are too powerful, compared to weapons. In this game that I need to write, I also need to lower spell damage, and make magic equal to weapons, and
much more subtle!
