Usability is not the only factor in a site's success, but it's the most important. If improving your site's usability isn't your top priority in your marketing plan, I recommend that you make it so today!
Attracting new visitors to your site is essential. But unless your visitors leave with a good impression of your site, they're never likely to return, which is money down the drain.
So, set the goal of improving your site's usability as a top priority, right now!