I'm not going to get into a long post about game design theory... I write way too much and getting into a super long post about that might bore the hell out of people. So I'm going to keep it simple.
Firstly, the most important thing about game design (in a professional capacity) in the end is understanding that any particular individuals game style is not necessarily the best even if it is the perfect style for THEM. There are things I would implement (and have) in my own games that there's no way in hell I'd try to integrate into RM as a published rule/mechanic. For example, for the round structure I'd either go with a BattleTech style round (simple, but better balanced than a D&D round IMO) or I'd go all out second-by-second with reach modifiers and speed factors, etc. But the reality is that most RM users will prefer a 'halfway point' between those. I think the RMSS round is the best round for RM as a published system - but I don't use it if I run a game.
My Tagline on another forum reads:
Fun. Balance. Realism.
In. That. Order.
Firstly, let me say that "Game design" in the context of this thread, IMO, is assuming you are trying to create a full fledged system that will be published to a large(ish?) customer base. Therefore you must assume you will have players of all experience levels and possibly even gaming style preferences. Although it could be argued RM could be marketed to veterans gamers who like lots of detail in its heyday that is not fully the case anymore. A well designed game system needs to appeal to as many people as possible to do well - or, really, just do more than keep it's head above water.
IMO there's no percentage of each of those (Fun, Balance, Realism) that needs to be maintained, rather it is a progression of which trumps the other when all are not possible. If you could create a game that was 100% realistic, was fun and well balanced then you've struck gold. There is no 50/30/20 or some such balance in my book. You want each of those things to rate as high as possible... but reality is that you're never going to have all three at 100%.
For example, is it "realistic" that an individual could bring someone back from the dead by praying to their deity, or transform into a dragon, or create a fissure in the ground with the wave of their hand and some chanting? Of course not. This is where fun comes first. We're playing a fantasy game (well, in my example). Realism is often out the window when it comes to fantasy gaming, but if you can have it in any many places possible, or you can give an explanation that creates plausible suspension of disbelief, without sacrificing the balance of a game... obviously you should do it.
Balance is important from a player standpoint mainly. If a system does not have some semblance of balance then there WILL be problems. No one here can convince me that a system which is not balanced will not eventually cause a breakdown of one kind or another... which is not fun. If anyone is familiar with RIFTS and knows what a "Glitterboy" is they know that it (last I knew) is utterly unbalanced when it comes to most other 'professions' in the game. And, yes, you can create a game where there is a balance mechanism to adjust for this (I can think of a couple ways, but I'm not going to get into that)... but the point is unless the GM makes a significant effort to account for the systems imbalance there's going to be conflict among many groups as a result. If you want your game to appeal to as many people as possible, and be fun, you have to assume most GM's you want to buy your system (cause you want everyone to buy your system) aren't going to have the experience to handle that kind of thing. So you make the system as balanced as possible without sacrificing the fun.
Fun is a no brainer. It's why we're playing, to have fun. If it's wasn't fun then we wouldn't be. Obviously fun can be subjective, but I think you'll find a fairly common, while somewhat generic, opinion when talking about targeting a large audience with a professionally publish game system.
The point is that if something is 110% realistic, but results in an unbalanced or un-fun system rule... then it's out. If something is realistic but unbalanced then it's out too because, odds are, it's going to create an un-fun situation. The game Axis and Allies was originally based on WWII actual facts/statistics/numbers. This resulted in a situation where Germany would crush everyone else a large majority of the time. The problem was that, statistically, Germany had a significant enough advantage in resources that they should have won WWII if all sides involved had similar levels of tactical prowess. We could get into a big discussion about that, but to put it in the most simple way possible: They didn't win because Hitler was a nut-bar. So when you pick up Axis and Allies you are playing an "unrealistic" game. Why? Because it wasn't balanced, which was no fun for anyone but Germany (assuming that player got satisfaction out of wiping the map with everyone almost every time). So they altered the reality of the game to be balanced, and therefore, more fun.
