From what I've observed, niche communities prosper faster and better (probably bcoz the web has evolved a lot and has become too competitive). For example, a community of owners of certain model of Nissan car will fetch a decent number of users, but my website - a community of MBA aspirants - is struggling to get past single digit registrations. I believe that's because there must be 100,000+ MBA aspirant communities on the web today, so my website gets buried deep in the Google search results. So if you are starting a community, and haven't yet finalized the 'theme', go for a niche area.
At a risk of digressing and probably starting an argument - I feel, the Google-Friendly URL rewrite MOD will help. It rewrites the page address to include the title of the post, instead of the plain - viewtopic.php?f=2&t=3, etc. Although this doesn't affect the efficiency of the Google Crawler, I feel it may get your site a better ranking, due to the weightage to URL words in calculating the page rank. Need proof? Search Google on 'mba rankings' (or any set of popular keywords) and notice that majority of the search result URLs have the search terms in them.
I'm planning to use the MOD.
Please understand that the efficiency of crawler is a different issue from page ranking, and do not reply with this forum's success on Google - how the posts get indexed within minutes and all that.