1. Flash based content. Flash does not work on a lot of mobile devices esp the Iphones. Make sure that the Videos are not flash based. Use HTML 5 tags.
2. Slow Page Load Times. Making your mobile pages load faster involves making sure that content above the fold is rendered in such a way that external JavaScript and CSS are not blocked. Bryan McQuad, does an excellent job describing how you can Make Your Mobile Pages Render in Under One Second.
3. Broken URLs are the worst. Test your website
4. Avoid Popups. Popups are very annoying and an average user usually complaints about them. Yet they are high effective for the Desktop websites. But you must avoid them for smartphones for sure. Because of the small screens and still not the most perfect touch interface, popups can make users leave your website altogether.
5. Avoid separate mobile website instead use the responsive website. This is also great for your SEO.
6. Easy to share URL links.
7. Avoid Small difficult to read font.
8. Dont use Images that dont look great on Mobile.
9. Make sure that the links and buttons Thumb friendly.
10. Most important is make sure that your phone number is clearly visible and can be dialed with one click. Very funny thing is that businesses who have spent thousands on developing the mobile version of their website (m.website.com), even they dont have their phone number visible on their mobile sites. Why would that be – dont you want your customers to call you?