I've changed my mind a bit on a certain subject within the realm of game design.
I've always liked the idea of a single coherent system that can be applied across everything in a game. But in a lot of game designs I've admired for this streamlining, the single system at the core is good, but it isn't necessarily great for everything. Trying to shoehorn the same system into all aspects of a game may work for a lot of the situations encountered, but might be a very bad fot in other parts of the game.
I'm not saying that you need a seperate subsystem for every element of play, and I know many games that have taken this approach... i'm just saying that I've come to the conclusion that two or three complementary systems that cover a wider range of situations effectively might be more user friendly than a single system that once one thing brilliantly and a bunch of other things adequately.
A combat system doesn't need to look like a lock picking mechanism, and neither of these have to resemble a method for social interaction.
If you're telling a specific type of story, where each of these elements of play contribute to the story in the same way, then it might work to use the same mechanisms for everything. But if you want a change of pace, or a varying narrative, then maybe it makes sense to resolve different things in different ways.
Similarly, if a specific element of the game is meant to be the signature point, then it might make sense to use a different system for that element of play. Some games seem to do this inadvertently, perhaps making combat a seperate system to everything else without realising that this produces a disconnect in the experience of the players.
As long as different subsystems are capable of communicating with one another, perhaps providing feedback loops that modify one another, so that they don't stand too independently of each other.
These thoughts have come from trying to integrate a combat resolution system into the same parameters bordering all other elements of a game, juggling the two extremes of a simple narrative system and a clever combat mechanic where hit locations, strategy, and different combat tools play a role.
Again...more to think about.