The most important thing on any website is a call to action button. It is very important that your user has the opportunity to buy, download files, subscribe to your website and share the relevant information. A Website that does not have a call to action button is simply useless. A user can read the information on your website, but what is the point if he is not able to make a purchase? The absence of call to action triggers or an illiterate arrangement of them decrease a conversion on your website which leads to a lose of money in your business.
Only use horizontal scrolling in extreme cases when it is appropriate. Horizontal scrolling annoys most of the users because they are not as used to it, as to vertical scrolling. Use a horizontal scrolling only if your content can not be placed properly on a screen. BUT remember that a good web-designer can always find a way to place information without using a horizontal scrolling.
Overuse of PDF files
PDF-files are useful when you need to demonstrate a large amount of information with an abundance of text. In such cases, it is better to create a PDF file rather than to make a separate page on the website with a plenty of text. BUT, the overuse of PDF-files is very annoying for users and you need to be careful when you use them.
Overload of audio & video content
Excessive use of video and audio files on your website can negatively affect the speed of its loading. Even though this is a great and very convenient way to transmit the information to your visitors, it is important to remember that you should not use more than 2-3 video/audio files on a single page. You should also bear in mind that audio files should not be played automatically to not disturb a user with a sound when he does not expect it.