We're starting to get that bug to try switching up our RPGs for a bit of variety. I've heard that a popular system, known as "Cortex", is used in several settings which our group would be enthused about, including Battlestar Galactica and Firefly. How big of a learning curve is there in this system? What are the main differences between it and d20/nWoD? What does it do better, or worse? What are the main "gotchas" that we will encounter in a transition?
I played the Serenity RPG a while ago, so I'll give you something that may help out. Hopefully someone who's played a bit more can give you a more comprehensive overview.
The system is your basic point-buy, skill based system more akin to Vampire than D&D (i.e. no classes). Characters have a series of general attributes (e.g. dexterity) and specific skills (e.g. firearms). These are rated by a category of die: d2, d4, d6, d8, and so on.
When you need to make a check, you roll the die corresponding to your attribute, and the die corresponding to your skill and add them together.
This does have the drawback that skill checks tend to be pretty "swingy." I believe this was mitigated somewhat by a fate point/drama die system of some kind.
For wounds, it uses a hit point system, with wound penalties as you take more damage. Hit point totals are generally pretty small (less than twenty, I think).
What stood out the most for me was how the system dealt with death and unconsciousness. Basically when you would die or be knocked unconscious you get to start making progressively harder checks to stay conscious or alive... It gives you that cinematic few moments of trying to accomplish something before the lights go out.
Also, doctors can save a mortally wounded person minutes after "death." Which is a nice change of pace from most systems, where a character bleeds out in twenty seconds or less.
Wikipedia article on the system is here:
The earlier games in the Cortex System (Serenity, BSG, etc) Are fairly traditional. (The first game using the engine predates the Cortex System Label: Larry Elmore's Sovereign Stone.)
The basic mode is stat+skill, roll and total, try to exceed difficulty. Each is rated with a die code, rather than a fixed value. This means that performance varies widely. Fate points can get extra dice, as well.
Opposed rolls are common; higher roll takes it.
Skills of d2, d4, or d6 rating are broad; d8, d10 d12, d12+2 are narrow.
The more recent design for the Smallville goes far afield whilst still retaining the same basic mode.
Instead of attributes and skills, it uses Pesonality traits and relationships... So it's more "what's important enough to you" than "what are you able to do."
Learning Curve: the system's generally pretty easy if you can wrap your head around rating things in die codes rather than straight numbers.
The character generation is by spending points.
The big gotcha is the widely variable results... don't make a roll if you don't have to, since since the performance is pretty wild. D10+d12 is great... but it's still possible to roll a 2... so the TN's can be deceptively hard. Especially since they're about 15 points lower than in d20.
There are a couple of ways that Cortex differs from more traditional games:
Other than that, it is a lot like a trad game.
The rules for CORTEX are quite intuitive: when a characters attempts to perform a task, the storyteller determines a numerical value for a difficulty, and -usually- two dice are rolled to determine whether the action is a success. So for example, driving at high speeds while avoiding obstacles in the road might require an Agility (Attribute) + Drive (Skill) roll. If the numerical difficulty set by the storyteller is matched or exceeded, the task is successful.
There's no marriage between attributes and skills, so once an action is described, the group, player, or storyteller can 'logic out' which attribute/skill combinations would be appropriate.
Characters can be further customized by purchasing traits: Assets (providing advantages) and/or Complications (providing disadvantages).
Vehicles -be they cars or spaceships- are handled much like characters, having both attributes and traits.
Size is handled well: from small scale and upward, damage is increased or reduced, at ratios of 1:10, 1:100, etc.
Plot points are awarded periodically to players, and these can be spent to get bonus dice, or to make changes to the story itself, making the game more collaborative in nature than what is typical in most other systems.
In terms of learning curve, it took our group very little time to get the hang of it (perhaps half of a session), and character creation typically takes about 10 minutes or so.
You can get the CORTEX system toolkit on MWP, which is referred to as CORTEX Classic. While this toolkit allows one to run virtually any sort of game, I find it especially effective for running sci-fi; I use it routinely for post-apocalypse, cyberpunk, and far future stories.
Seriously, if you pick it up, you will love it in all likelihood. The system is rules lite, intuitive, and award winning.