1. I would move the log in box to the upper right hand corner. ...Same with the "register".
2.I would enlarge the search box and make it look like a fill in search box and not a grey button.
3. I would add a tagline under your site name. I don't really know what you do- maybe your customers do, but I don't. I gather that you sell car parts of some sort.
4. Your featured products-aren't really featured. The drop down tab is barely noticable.
5. I don't care for the duplicate info running vertically on the left. Reason being, 1. it's confusing, and 2. at first it looks like the info is duplicate, so I skimmed it thinking it was that same that was in the nav bar- but it's not all the same.
6.I would move contact us and FAQ to a more prominent position on your page.
7. I would add a copyright and privacy disclosure at the bottom of the site.
8. Personally, I like how you have positioned your products within the online store. ...Is there any reason you don't have them set up like that on the home page? (With the price, ratings,etc.)
9. The vehicle Specific Blowoff runs off the page in IE.
10. Your continue shopping and checkout is nicely laid out and easy to use.
11. Your site does a good job with breadcrumbs.
12. I don't think you need a home/contact us in the upper left hand corner.
13. I would throw in some pictures of people. Maybe pictures of your team standing around a car with the hood open. A picture that says that you know and love what you are doing.
14. I don't quite understand the difference in your tab system. It looks like you give people the option of going to your online store, yet, then you have tabs next to the online store tab that are the products featured inside your online store.
15. I would replace the vertical nav bar content and either just leave contact us and FAQs. Or have Contact Us, FAQs and testimonials on each page. ...Ideally, product specific testimonials per product page.
16. As far as I can tell the online store tab is redundant. I would remove it, and let people just search for products via the search box or by tab. Since you seem to specialize in certain parts, I would leave them labeled at the top like you do.
17. In the review section, items are coming up as having 5 stars, when there is no review to back it up. I would have the default be 0 stars.
18. Another thought is to maybe add a forum down the line. It looks like your product fits a niche, and that niche is passionate about cars. I would use a forum to generate traffic and repeat business.
That's all I see at first glance. Good for you for taking action! ++rep