The rules are meant to be easy - I prefer to keep the game mechanisms as simple as possible, so they don't themselves become the focus of the game, but tools to make the story flow.
However, although I'm using the core system I've employed for several sets of rules, modifying them properly for a new genre is a task in itself. I dislike rules that appear simply to have, say "tommy gun" crossed out, and "amazing multi-shot crossbow" types in, and are then called complete. The worlds of Swords and Sorcery have their own special flavour, and the rules ought to bring that out.
As an example of that, I'd point to Gloire as a successful use of a base system modified to fit the period, rather than the (too-often-seen) alternative where the period has to fit what's already in the rules. At no time playing Gloire do you feel, "Wait a minute! My musketeer just sucker punched two goons and fired his pistol at a racing coach --- this is just a gangster movie in weird costume!" :
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