Smartphones and tablets should be your site's first priority from a design & development perspective. Clean layout, proper use of white-space and proper font sizing are key factors in usability.
Clear call to action:
Does your site have a clear message that's delivered in a few seconds? It may not be necessary, but if you're selling a service or a product you want to make it quick and easy for a potential customer to take an interest in what you're offering and why you're a better choice than your competition.
Easy to find contact information and easy to use forms:
Nothing sucks more than having to click through three links just to get a phone number, email address or contact form. This information should be present in the site's header or footer on pretty much every page. If you're using an online contact form, be sure it's simple and easy to use. Too many required fields and a poor layout will most likely lead to users abandoning the form completely.
Be social - within reason:
Social media tools can be beneficial to your business, but don't feel you have to use every single platform that's available. Use only the social media tools that you're comfortable with. Be sure to maintain the account - if a potential customer sees that you haven't made an update in over a year, they may use that to form a negative impression about your business.
Ditch the Flash introductions (and Flash in general):
These intros get in the way of delivering any type of call to action, and they're not mobile friendly.
You vs. your competitors:
How does your site stack up against the competition? What are they doing that you're not? Does your site make a better first impression?
Up to date platform & plugins:
Updated CMS frameworks like Joomla, WordPress and Drupal are important in keeping your site and your customers safe and secure. Plus, new platforms always offer the latest features for both visitors and site administrators.
Fast, reliable hosting:
Everyone can agree on this one - slow loading pages suck! Web surfers are an impatient bunch. If your site takes forever to load due to your web hosting company, it's pretty much a guarantee that the user has already clicked their browser's "Back" button and have moved on to another site. When it comes to hosting companies, you truly get what you pay for! (Side note: keep your website image file sizes small.)
Image reference (with great "what not to do" tips): Webpages That Suck