PrinterIsOnFire

Because it's not my printer. It's my ex-neighbours' wireless printer.

What RPG system mechanics do you particularly like and/or dislike, and any reasons why?

I spend a lot of time messing around with them, so I always like to hear what other people think.

Here's a website you may find useful: anydice.com. It's a probability calculator for dice mechanics, and as long as you can handle the pseudo-code it uses for input, you'll be able to make it show you all sorts of complex dice odds, which helps if you're trying to balance homebrew mechanics. You want to test a system of "Your skill is rated at two dice between d6 and d10, your stat is a modifier between -4 and +4, you roll both skill dice and add your stat modifier, and if one or both of your skill dice rolls equal to your stat modifier you add an extra d4 to your roll."? You can use Anydice to output the odds of that theoretical (and fairly weird) system and work out what the average for each triad of stat and two skill dice would be, with and without the Bonus D4 thing.

I cut my teeth on D&D3.0 and moved on to 3.5 for quite a long time. In the past five years or so, I've experimented a lot with systems and found I prefer games with rulesets that allow for more flexibility, as my DMing/GMing style is "fly by the seat of my pants, and always have two interesting options when a player tries something important. Whether they pass or fail, the game should move on."

As such, the strengths of the various D&D incarnations - highly tactical combat that benefits from planning encounters ahead of time - is not one that I mesh well with. I've enjoyed all the D&D I've played and run, but I feel that my narrative games are probably better experiences for my players than any D&D I run.

My main "complaint" with D&D3.5/PF is the degree to which the random and fixed parts of the system change with relation to one another (that is, at low levels the d20 you roll has more impact than any possible modifier you have with the single exception of the True Strike spell).

At the moment, my two systems of choice for homebrew / system hacking are Unknown Armies and Cortex Plus. UA is Percentile Roll Under, with four stats averaging 50 each for "normal" people, and skills being categoriesed under one of those stats, and a skill of 15 being "competend enough to do this under normal circumstances". An unimportant task is automatically completed if you have a skill of 15. A medium-difficulty task is completed if you roll equal to or under the Stat that governs the skill, and a high-difficulty task is completed only if you roll under the relevant skill. In this manner, a normal person with Speed 50 and Driving 15 can drive to work every day and be fine, has a 50-50 chance of dinging their car if they're driving hazardously in a hazardous situation, and only a 15% chance of not screwing up a high-speed car chase on an icy road - and the player can tell all of this information without having to do any further maths or remember the DC for a given event.

Cortex Plus is more of a system-building framework than a game system on its own, but the basic explanation is this: Things are rated with a die type, from d4 up to d12. Ratings are their power to move the plot along. What exactly your "stats" are will vary between games, because those stats will be the things that matter. In a game that's all intra-party drama, how strongly you care about other party members will have stats. "I don't just dislike that wizard, I Loathe His Every Living Moment d10, and I would gladly take a bullet for Princess Elfstar d10, too." Cortex Plus is probably a bit less narrative than the FATE system, but it's close.