You say you`re getting 1,000 visitors/day. Do you know any more details about what the visitors are doing--are they looking at more than one page, how long do they spend reading each page, etc.? A good traffic tracking program will help you know that. They usually come as part of your website hosting.
I also had a similar experience to Craig`s. I got the impression from your post that your website was sort of like Consumer Reports--charts comparing different vehicles. But when I saw that I`d actually have to do something major to get to this information--either pay $25 or come back regularly to post info about my car mileage--I was no longer interested. If that`s how you want it, great. But it would have been helpful to make it clear near the top of the page (I was on the reliability page) that this was more of a club than a general information site.
One little thing: your line about "Current panel size: Over 11,250 vehicles" is strangely worded in my opinion. I think of a panel being composed of members, so this suggests the cars are the panel members. It gives me this odd mental picture of a bunch of cars sitting around in a circle, discussing other cars.
Also, the question about "if a few complain, what do the others think." What I`ve read in a few marketing books is that the minority will speak for the majority when it comes to problems, and you want to be really grateful for those who do speak up. Many people who have problems with your company just go away and never say anything, so you don`t know why you lost them.
As far as navigation issues, I think things would be helped if you emphasized the idea that this is a membership/club website, like I mentioned before. The "sign in" link should be prominent on the home page, not buried to the side, or have the sign-in spot right there. Pages aimed at browsers/newcomers should be somewhat separate from pages aimed at members. Also the "Think Pieces" button confused me. I thought it was talking about pieces of a car, until I clicked it. "Articles" or "Editorials" might be better.
Also, emphasizing the "club" idea might help you get those non-responding 50% of members to take more pride and interest in "their" organization`s efforts.
when results matter