I think we should aim to cover three basic things:
LEGO Bricks. Fundamentally, this toy dominates the building space. If we don't have this, we don't really have a site.
LEGO Not-Bricks. Mindstorms, DUPLO, games, LEGOLand, Bionicle, Modulex, tables, carrying cases, alarm clocks, minifigure collections, company history, BrickMaster and MBA, Znap. The overlap is enormous between people interested in these and the bricks themselves and it's hard to separate them - is a question about a model at LEGOLand a question about bricks, or a question about LEGOLand?
It's also important to cover this so the site can serve as a resource for parents (and whether we want it to or not, it probably will this Christmas).
Other Bricks. Diablock, Mega Blok, Nanoblock, and undoubtedly a new system every few years. Compatible or not, all brick toys share similar building techniques. In some cases, like Diablock, they are "not compatible" as stock pieces but people have built adapters.
That means we don't cover K'nex (perhaps shared interest but no shared building techniques), Meccano (in my experience, very little shared interest), and other progressively more marginal building systems. Otherwise I fear we'll see a gradual descent until we cover things that are barely even building toys like Domino Rally, and eventually just be a generic toy site.
You might say "I'm as uninterested in alarm clocks and Znap as I am in K'nex!" The good news is, the rest of the world agrees with you - the potential number of questions about these things is very small and can never come to dominate the site. That is not true of K'nex.
Why is it bad to offer more? Focus. If the site is 90% Lego, we're not going to attract K'nex or Meccano experts. If the site is 30% Lego, we're not going to attract Lego experts. We need experts to survive. Otherwise people will just go back to the existing Lego forums and wikis, because their questions can be answered just as poorly there.