On the

Remember that the chief question you need to answering (especially with Medieval fantasy) is, "Why shouldn't I just play D&D?" If your adventuring guidelines are too simple and bland, you aren't going to answer that question.

This is a very good guiding question. There is a complementary one, providing an equally important constraint from the opposite side: why shouldn't I just play Trollbabe? Does the combat rules system you developed actually pull its own weight? It might be designed competently, but is it fun? Is it better than nothing? (By "nothing" I mean for example the success roll system in Trollbabe, which is a reliable source of dramatic failures without any pretense of simulation and realism.)

I firmly believe in designing roleplaying games starting from how you want the act of playing them to be (i.e. from what players will like to do), and adapting setting and rules to that unifying purpose.
For example, D&D is the traditional market leader in intense and rather lethal, tactically complex and brutal fantasy combat: other sides of playing experience are sacrificed (e.g. such combat can take a very long time to play out) or offer important synergies with the main ones (e.g. character building to maximize combat resources) or are a necessary evil (tactical depth requires complex rules and a lot of content, creating opportunities for exploitation).
D&D rules have a very high complexity budget and they spend it all on combat: modifiers that can give you an edge, special things to do in combat (including spells), injuries, etc. Noncombat rules are rudimentary and/or bent to resemble combat rules and be compatible with them (e.g. assigning a "challenge rating" to a lockpicking task).
The default D&D setting assumption is variety, which is good for keeping players on their toes and allowing the DM to fit almost any kind of combat situation in the same open-ended campaign, as evident from the unusually large amount of monster races, alternate planes of existence and magical items and the penchant for "patchwork" official campaign settings (Greyhawk, BD&D Gazetteers, Dragonlance, Forgotten Realms...).

The diplomacy-type roll playing is going to be done by out-rolling a number, the number being decided by the GM. I was planning on having a sort of haggling over said number as part of the mechanic: if a player can state a good reason why he should have an easier time interrogating a prisoner than the GM thinks, they can have a discussion about it. The GM will set up outcomes for critical successes and failures.

Discussing modifiers with the GM is perfectly normal in any rule system that tries to be a fair simulation; leaving the definition of varying degrees of success and failure to the GM is equally obvious.
Many games neglect the possibility of multiple outcomes and/or try to define standard effects for critical successes and failures, but it is an obvious simplification and special case of making the best possible die roll. Is the flexibility in defining die roll results a particularly important part of your game? Why?

The mechanics include stored "Extra Rolls" that can accumulate, so if you don't like a roll, you can burn an Extra Roll and roll again.

Already done many times, and usually for specific purposes like giving players control over randomness, at a price, to avoid "unfair" failure (e.g. the various ways to burn character points on one-shot actions in GURPS) or providing a resource to care for (like the Perversity Points that Paranoia XP players need to obtain from the GM and spend to affect each other's roll). What is the purpose in your game? Where do extra rolls come from, if you don't have a normal experience system?

There are 7 playable races, and in lieu of classes, 3 "types:" Strong, Agile, and Magical. There is no level/experience system. To create a character, a player picks race, type, armaments, and magic(if applicable).

No stats beyond Type? Aren't you going to leave too many details implicit?

I'm giving the GM the option to ignore any of the adventuring guidelines, as what distinguishes my system is the 3d6 mechanics I've set up...there are different outcomes for rolling doubles, sequentials, trips, etc.

A rather peculiar element, which needs a special justification. Why is complicating die rolls useful in the context of other rules? How is it fun, or funnier than different sorts of complication like tracking exact miniature distances on graph paper or optimizing the weight and speed of detailed mecha designs?

Basically I'm trying to center the game around the narrative by keeping mechanics simple, and leaving most of the world-creating decisions up to the GM.

I'm afraid this is a bland and vague value proposition.

First of all, simple mechanics cannot be considered a good thing from an abstract point of view; specific aspects of specific games can be managed with simple rules because they are enough for that game's needs, while other aspects need more complex rules.

Second, if you "leave most of the world-creating decisions up to the GM" you are implying that your game is general and easily adaptable, with elegantly universal rules, at the expense of doing something special particularly well.
There are many games in this category, more or less semi-specialized for a broad setting type, ranging from simple (e.g. FUDGE) to medium (e.g. orderly skill-based systems like BRP and Fuzion) to complex (e.g. GURPS, D&D, Rolemaster).

Are you attempting to provide an alternative generic system with better rules (or at least novel ones), or are you combining a specific attitude or style with setting obliviousness (examples: Toon, Microscope, Fiasco, Primetime Adventures)? The only thing that you are actually stating is that you don't want to sacrifice generality to tightly integrate specialized rules with specialized settings (e.g. Ars Magica, My Life with Master, Pokéthulhu, Castle Falkenstein...).

The game actually began as just a gambling game centered on my 3d6 mechanics, when I thought, hey, let's build an RPG around this.

I don't see anything gambling-related in the rules you describe. Gambling might be a good idea (a setting of fantasy gamblers? Gambling between players on in-game events? Replacing simple die rolls with proper gambling games like poker? There are limitless possibilities) but you don't seem to have focused your design around this.

[quote name='mekk_pilot' timestamp='1339005563' post='4946818']
The game actually began as just a gambling game centered on my 3d6 mechanics, when I thought, hey, let's build an RPG around this.

I don't see anything gambling-related in the rules you describe. Gambling might be a good idea (a setting of fantasy gamblers? Gambling between players on in-game events? Replacing simple die rolls with proper gambling games like poker? There are limitless possibilities) but you don't seem to have focused your design around this.
[/quote]
I don't think he meant that the rules were in any way related to gambling. I think he meant that he originally created a gambling game with unique 3d6 mechanics, and to which he later added RPG elements. To the unique 3d6 mechanics, not to the gambling.

And hey, where you said...

A rather peculiar element, which needs a special justification. Why is complicating die rolls useful in the context of other rules? How is it fun, or funnier than different sorts of complication like tracking exact miniature distances on graph paper or optimizing the weight and speed of detailed mecha designs?

I don't think unique 3d6 mechanics needs special justification. It's like Uno. Uno takes "basic card mechanics" and makes it...funner. Just tossing the dice with special rules can be loads of fun. In and of itself.

Anyway, sounds cool mekk_pilot! Keep us posted!

Take care.