Note that Snark's First Law of Gaming works both ways. Telling a PC what can't be done is often important. For example, I wouldn't use Charles Ryan's Description-Based proposal to run a MAGE: The Ascension Chronicle. M:tA is all about Limitations, and overcoming them. Its more formal system provides a framework for the milieu. On the other claw, I wouldn't use GURPS to run a MAGE Chronicle, either, although the licensed supplement IS available. GURPS is far too rigid and reductionist a system to properly reflect the fluid nature of a Mage's Reality.
Example: Alan Dean Foster's insectoid aliens, the Thranx, are described in GURPS Humanx as a 0-point character race, which makes sense: the whole POINT of Foster's work is that while Humans and Thranx are psychologically and physiologically =different=, neither race is superior to the other. Any attempt to apply the point system of GURPS Aliens (et al) to the description in Humanx yields a race with a base cost of around 80 points -- when a typical GURPS campaign has a 100-point base.
Example: Try to create a CHAMPIONS or GURPS Supers character with the ability to detach body parts. It's easier in CHAMPS, but, even so, a character with such an ability will be easy prey for a super with a more conventional power set.