The big question. There are hundreds of rpg systems and some of them work better than others. Still, what games I don't like several people find great. It is just a question of taste. There is no ultimate truth for good system.
This is just a personal taste, what makes a good system. Everyone has their favourites and those, they don't like as much. And even those they hate. I read this* blog, and got idea to share my description(s) of good rpg system.
Easily learned rules
You learn rules fast or at least are able to play quite quickly even if you aren't familiar with all the rules of the game. I don't prefer very complex rules but rules, you learn easily and can get to gaming part quickly enough.
Rules easy to adapt and solid
If there is different rules for everything you do in rpg, remembering them can be quite hard. Optimal is when rules mechanic is as solid as possible in every area of the game. Also preferred is that if GM doesn't remember how something exactly works by the rules, he can easily fit the rules he already knows for the current situation. This ensures, that the flow of the story is smooth without unecessary breaks flipping through the rules book for some particular game mechanic.
As obvious as it is, proofreading is really important. It doesn't affect the system straight, but can make serious difference in rules. What I hate most are the rules where is several typos, indications to pages or tables that doesn't excist and so on. So I think, that well proofreaded product is also a good basis for working rules.
Fun to play
This is harder to define.ämä on hieman hankala vaatimus. How can you say what is fun to play and what is not? You just know and feel it. There is no real mechanic to define it. It is just a opinion. Some games feel right, some doesn't. Sometimes even totally different kind of games could both be fun to play in your opinion.
Not great amount of calculation before rolling dice
For example, if there is huge amount of different variables, bonuses, minuses etc. before you actually get to roll the dice to determine your success, it is not fun to me. When my character does something, I want to roll the die and see did he success, not spend several minutes before calculating every possible odd. If there is variables in system, I prefer them pre-calculated on character sheet, if possible. And odds I like in simple one variable, not 10 different you calculate together to get this one what affects your roll.
If all the previous needs are somehow filled, I don't actually care what or how many I roll these little buggers. I like Fading Suns where you use one type of die. I like Storytelling system where you roll fistful of dice of same type, I like BRP where you use several dice. I like dice. They cannot cause problem, I quess.
Book keeping not so detailed
Over detailed book keeping can really be annoying. I personally want to focus on adventure and story, not on how many items my character can carry and stuff. I don't want to spend my gaming time writing numbers with my pen. Also in some games combat book keeping can be annoying. You write down turns which everyone does his action(s), but sometimes different actions give minus for initiative, so you need to re-calculate and so on. I like fights as action, not figuring out who does what and when with numbers involved. But I am not against initiative in general. I just like it simple. See who does what and when and do it.
Experience and characters advancement are fun. I am not sure, what would be my preferred experience system. I got nothing agains level based system, but still also buying skill points with experience points work for me. I am not too picky with experience system, but one criteria there is. I want to see my character advance. For example BRP experience system Isn't the best I know. You advance really slowly, and when you do, few % doesn't make much of difference.
I always like, if there is some witty idea in system. For example Praedor is very simple system but I like it's "deep wound" rules(1). I also like Fading Suns Victory chart. And of course I have to mention my own system Ydin's "Over/Under" system.
Advanced and optional rules
Several game has basic rules as text, but optional and advanced rules are presented for example in boxes. You can freely and easily choose, what of these optional rules you use, or can start to use them when you are familiar with basic rules. It makes it easier to learn rules, when you see what you most definately need, and what are advanced and not necessary to run the game.
(1) In Praedor, there is characteristic called "Deep wound". Every damage what goes over character's deep wound number is concidered as critical hit. If for example character's deep wound characteristic is 8 and damage is 11, you refer deep wound chart from column 3 to see the additional result.
Originally posted similar topic in roolipelit.net forum here. I just translated and slightly edited it poorly into blog.