I'd lean toward a liberal standard for "local" flavour proposals, an in-between standard for "global" flavour proposals, and in-between, maybe even on the restrictive side, for crunchy proposals.
By "local" I mean features that don't impinge on much of the rest of the world -- maybe developing some town or small area of the map in detail, although it doesn't need to be local in the geographical sense. A organization that exists worldwide without being such an important player as to substantially shape the culture of the world (e.g. the Freefolk) would also get the liberal standard from me. If you've got an idea, the world should be big enough to find space for it.
A "global" flavour proposal could be a new god, or a major historical personality who shaped the world in definite ways. Of course, I want to be open to ideas if they do fit into the world as it is developing. But we do need a little restraint in the interest of coherence. Create a god of fire (for example), with all the associated mythos about how and why fire was given to the mortals, and the rest of the mythology needs to fit in with it. By all means, we want to encourage that, so I still want a fairly liberal standard, but we ought to consider how a proposal might restrict future ideas. That's not a problem with local proposals.
The reason I favour a stricter standard for rules-driven proposals is that I don't want to end up with a world that seems too foreign to newcomers familiar with standard D&D. Any time you join a campaign, you expect a lot of world-specific geography, organizations, culture, etc. that you hadn't been prepared for. But you also expect seven core races, eleven core classes, etc. If a new player brings his first LEW character into the Inn, meets five people of five different races of three of which he's never heard of before, it might be a little off-putting. Some people are happy playing every game in a totally different setting, but some people prefer to get to know the rules and options of their system inside out, and I'd like LEW to be accessible to that kind of player too.