RPG background
1. Took the Fallout 2 rules handbook and designed a pen&paper version to play online. At that time, Fallout PnP hadn't been developed yet.
2. Decided to make my own rpg, at the time called "Blades of Gnisis". In retrospect, it sounded really stupid; I was 15 at the time give me a freakin' break! I changed the name shortly thereafter to "Nevercast" because I thought it sounded cool. To this day, I've never bothered to change it, so instead I thought up of an explanation for the name. Considering all the research I have to do for the mechanics, I'll wager it'll be another 3-5 years before I actually finish the damn thing.
3. Came up with an idea for an rpg/story in which anthropomorphic creatures represent personifications of the mind.
The Importance of Mechanical Structure
In the process of my creations, I've devised many different mechanical forms. In my opinion, the mechanical form of your rpg should fit the following criteria:
1. The majority of the content needs to be original, or is inspired from so many eclectic sources that the composite creates something original in of itself. Furthermore, when you compose your rule set, some of the first questions you should be asking yourself is, "How will this stand out from the others? What will make this system fun and engaging?"
2. The mechanical structure needs to correspond accordingly to the rpg style. Storytelling rpgs seem to do better with generalized mechanics and Wargaming rpgs do better with detailed mechanics.
3. No matter how detailed the mechanics are, they should be as streamlined as possible without sacrificing systemic integrity. Strip away mechanics that are superficial or have a minimal impact on gameplay. Much of this streamlining can be accomplished by keeping your descriptions short and to the point.
Storytelling RPGs
I have the most fun devising the mechanics for these (I've many scrapped projects that weren't previously mentioned). The reason why is because I can spend more time thinking outside the box for fun ideas and less time worrying about the realism. For example, in my 3rd listed rpg, since it's based in the mind, the player characters don't even possess attributes. And since nonsensical, arbitrary concepts like "physics" aren't a concern, I can pretty much design my setting and mechanics without any constraints whatsoever.
Simulationist/Tactical RPGs
I made the mistake of attempting to make one of these when I started out as a 2nd edition D&D nerd in my teens. From my experience, this is by far the most difficult type to design mechanics for. The problem is, humans aren't computers, and we can't process countless calculations the way a computer can. And since this type of game requires a higher level of complexity than the others, I often reach a design impasse because the streamlining process compromises complexity. I find that most of the current mechanical structures are inadequate for achieving this effect, so you're going to have to figure out a clever way to make simple core mechanics have a large spectrum and fine granularity of results ("Godlike" is one of the best examples of this). If you're going to tackle one of these, my advice is to start out with less restrictive rpg forms.