Rules suck. There we go, I've said it. I hate roleplaying systems. Now, it's a love/hate. I love game design. I love seeing cool mechanics. I love neat dice tricks and bell curves and character balance and generation. What I hate is the effect rules have on the actual roleplay. Too often, I've been on the receiving end of rules lawyering where the game stopped mid scene, because someone "needed to know" exactly how X worked mechanically so that they could use X kewl power to defeat X baddie. The rules should play a second fiddle to the story, but often they overshadow it completely.
Rules influence how we talk about the games. People say "I play AD&D" or "I play Burning Wheel" or "I play GURPS." What they should be saying is something along the lines of "I play a young wizard and we're on a quest to bring down this evil empire..." Screw what system you're running. What was your game? I tend to dislike playing D20 Modern, but one of the best games I ever participated in was a space opera using D20 Modern/Future rules. It was a formative game of my experience as a roleplayer. It (usually) transcended a deeply flawed system. When I talk about the game, I talk about the characters (I was a plucky, quirky young mechanic who fell in love with the robot girl who was our computer's AI), not my feats and stats.
The rules cause schisms in the community. At the comic shop I played at during high school and college, there was a deep enmity between the old timers who played AD&D 2E in the back room and the younger kids who played D&D 3.5 in the front. There were one or two people who crossed over, but most of the people I play with even to this day won't touch a 2E book. Heck, I even felt that way for a long time, although that was mostly over the fact that I couldn't figure out THAC0 (which I recently remedied). With the right players and GM, roleplaying is roleplaying. The story can be the same in Traveler or D20 Future as long as you let it.
Now, some games are best served by certain rules sets. I would hate to run or play Vampire: the Masquerade with a D20 system. I would play as long as the game was good enough, but it would be hard to not just say "guys, let's just switch. I have V20, and I'll do all the conversions for you." White Wolf is one of the prime examples of this. I've messed around with the GURPS versions, and they really don't feel the same, but again, as long as the game was good enough, I'd play. Conversely, sword and sorcery is served equally well by D&D and say Burning Wheel. The more specialized the world, the better it's served by a system created specifically for it, but if you're playing a more generic game like "space opera" or "horror" or "sword and sorcery," don't get too hung up on which rules set you're using.
I'm trying to drive this home to my current group. We played D&D 3.5 almost exclusively for years, only switching to Pathfinder under much protest. When I tried to get some GURPS 4E thrown into the mix, I met with highly mixed reviews, a few outright refusals, and I eventually just gave up. To their credit, we've moved away from D20 based systems a bit lately. I've been running a Hunter: the Reckoning game with quite a bit of success, and my roommate is hopefully going to run the D6 A Song of Ice and Fire RPG for us in the near future. They've been receptive to me mentioning running both Savage Worlds and Burning Wheel as well. It's exciting.
However, I haven't run those games yet for one distinct reason: I don't have a story I'd like to run that jives with either of those systems. Heck, my biggest hang up with my Hunter game is that the World of Darkness's rules and lore are bogging down my current game. One of my players played quite a bit of White Wolf back in the day, and he knows what I'm pitting them against within usually a few minutes. I've been stifled creatively, because out of laziness, I'm falling back on pre-scripted monsters outlined in my large World of Darkness collection instead of running them as I would. I only became aware of this when I tried to make some slightly Chtulonic beings and just gave up and used Demon: the Fallen, because I had the book on my shelf. I've loved those NPCs (and more importantly, the players have too), but I feel guilty, because I let the pre-scripted plot and rules write the game and not me. More than once, I've wished I ran the game with GURPS or Savage Worlds so that we didn't have the World of Darkness to contend with, and I could make the shadows unknown and thus more frightening.
In a way, rules are important, because they do shape the game. From what I've read, I'm glad my roommate is running the Green Ronin D6 version of the Song of Ice and Fire RPG rather than the D20 version published by White Wolf. I appreciate the difference, mostly because we're probably going to be playing with some new people who have played D&D before, and I think the different system will help break them of a hack and slash mentality. I'm also glad he's using the actual ASoIaF book over say, GURPS, so that we have world specific options, as I've only watched through season two of the TV show and haven't read the books, and one of the guys hasn't even watched the show. It'll help us get into the world since we're picking from options that are already there.
What I'm afraid of though is that we are going to get hung up on the rules. I don't know how the system works. My roommate is the only one with the book, and he's going to be the only one who knows how the game works. I plan on roleplaying the hell out of it and just forging ahead, but most likely there are going to be game flow interrupting "rules breaks" during our first few sessions. I will be annoyed, but hey. It's a game, and I'll be playing with my friends, so it'll hopefully be a good time.
When I run, I fudge rules for purposes of story a lot. My epic boss just got one shotted because of a rules glitch in a broken Pathfinder build? No prob, he has 100 more HP. The fight rages on, people do epic things, and my player's have a more memorable combat versus "yeah, the boss went down like a bitch because there was a charge lane and I got in 200 damage on a regular hit." Most of the time, the dice fall as they may, but if the rules are standing in the way of the session, screw them. Roleplaying, to me at least, is about a story, not about chucking some dice and playing with probabilities.