Let's not forget about the most important factor of being in business in the first place, (ROI) Return On Investment.
As you already know (By the information originally posted by the thread starter and other members posts), Link building, Content, image optimizing, on-page optimizing, etc... are the starting factors.
Now lets forget about the search engines for a second since you've already optimized the site & focus on optimizing for viewers. One can sit in the #1 spot of google for a popular term generating 20,000 hits per day, but if the site itself has little interest to the visitor, no conversion takes place.
With that in mind you also need to understand that after years & years of research, the average visitor will make a decision to continue viewing a site or leave within the first 3 seconds of landing on the page. This means that the landing page needs to capture interest immediately to achieve retention.
Retention is the threshold for ROI. The longer you retain their interest, the more they start to trust you, & the more comfortable they become in the conversion process. This is where a stimulating design, complementing theme graphics/images, & "call to action" phrases come into play to complement the site content already drawing the traffic from your seo efforts.
The main key is targeted traffic visiting, meaning the people coming to your landing page already want what you have to offer & aren't just random hits (Something you already achieved Via your organic seo efforts & strategically targeted marketing / advertising campaigns). A strategically targeted campaign makes all the difference in the world.
I could go on & on, but I'm sure you get the idea & direction I'm going with this.
Hope that helps, good luck!