I always figured that the most important parts of game design are just like any other design methodology. You set yourself some ground-rules, then you let yourself run while with those core factors as a grounding point to measure progress against. I do this with a process of defining positives and negatives, then dividing these into negotiables and non-negotiables.
Positive Non-negotiable: The design needs to accomplish these before it can be considerd successful.
Positive Negotiable: These be great if the design accomplished them, but they aren't deal breakers.
Negative Non-negotiable: If the design strays into this territory, it's an instant fail.
Negative Negotiable: These aspects don't destroy the design, but we'd rather not have them around.
The catch is, most people have a different list of negotiables and non-negotiables, and some people vary between their interpretations of positives and negatives...that's probably one of the reasons why there are so many game designs catering to the wider community of gamers.
John might like figures (they're a positive negotiable for him), and he might hate numberless trait-based games (they're a negative non-negotiable).
Svetlana might think figures detract from the game (they're a negative negotiable for her), and she might have a tendency toward numberless trait-based games (they're a positive negotiable).
Maria might come from a wargaming/boardgaming background, and might only play games with figures both in the miniature sense and number sense (both are positive non-negotiable).
When we look at designing any game, we need to consider what factors in this list will help contribute to the play experience we are trying to achieve, and also what factors might align more closely with our designated target audience.
Here's what I'm thinking for the 'Boffer' LARP system.
There needs to be a system for tracking conflict through bashing one another with padded weapons.
It needs to be quick, avoiding the need for books to be carried around.
There needs to be enough autonomy in the characters and setting that a GM doesn't have to be present all the time.
There should be a magic system of some type.
There should be a few ways to make characters distinctive. This might be through races, occupations, cultures, factions, background options, or something else that we haven't considered yet.
It should be convenient enough that most rules can be remembered by most people. (Maybe a limited exception based design).
There should be an ecosystem, an economy within the setting and the rules. Something to perpetuate stories and narrative, triggering new ideas through the actions of players and characters.
There should be a system for character advancement, and possibly some system of benefits for players who contribute outside the game.
Everything needs to be capable of occuring in 'real time'. We don't want to break the action over there so that we can spend a few minutes over here resolving something that should generally be instantaneous.
Avoid the system getting to complicated and clumsy. It should be welcoming to new players.
Perhaps avoid dice, because it's inconvenient to roll them in the middle of combat, and sometimes just hard to find a flat surface to roll them on (revealing cards might be quicker/easier, maybe something else).
Avoid 'perfect builds' where certain traits/skills/effects combine with others to give massive advantage over those who don't possess them (we want variety in the characters).
Players shouldn't be able to disrupt the pleasure of others without feeling consequences.
Some people might put the presence of a magic system into the positive non-negotiable category, or might drop the character autonomy to a negotiable...but with my background in Australian Freeforming and my experience with both 'Boffer' and Parlour LARPs, this list feels like a good starting point for a fun game design. Notice that I haven't bothered looking at numbers, mechanisms, possible races, or even themes in the game. Also notice that there are more negotiables than non-negotiables in the lists. I'm not saying this list is perfect, it's just what I'll be aiming for in this design.