Quality Content + Usability = Success
Quality content alone will not help a site grow. Quality content can help draw people to the site; but, the overall experience is the key to acquiring and keeping quality members.
What you describe as crap others may describe as quality.
Your forum's style (layout/design) can have a (positive or negative) affect on seo and growth. Based on what I have seen so far, 99% of current custom vbulletin styles have a negative affect on seo and usability.
A quality member is defined as:
someone who registers and participate (not leave after a few weeks use)
someone who participates frequently (active)
someone who brings in more members
produces quality responses, articles, etc
Here's some keywords and it's google rank: (look for vb.org)
Cache Templates - #1 out of 1,490,000
Template-Based System - #1 out of 843,000
Create Custom Pages - #1 (generic) out of 28,200,000 unexpected
Custom Pages - #10 out of 99,200,000
Development Cycle - #37 (generic) out of 91,100,000
Woops, looking at these Google results tells me there's a few things I can improve on. FYI: Search Results could be better (throughout the site); but I ... never mind.
My point is .. if you are really worried about growth, you should spend more time on developing a site plan and style that is both user and seo friendly.