Don't force users to install strange BHOs. Many tech-savvy users won't do it. Common offenders include unusual video players, image viewers, and platforms for interactive games. For most standard use cases, there is a trusted plugin, such as Windows Media Player or even Flash) that will do what you want without driving away users.
Build your website using a content management system. Knowing how to build a website using just HTML and CSS is good foundation knowledge, but this can only produce a static website. Web design has evolved into providing dynamic content. If you couple your coding skills with the use of a content management system, you can practically build any type of website that you desire.
Do not go overboard in using lots of fonts when creating your website. You definitely want to consider what the fonts are actually going to look like on different screens and whether they are too small to read easily. Verdana is used by most sites. It is easy to read in various sizes and colors.
Include a links page for your site and use it to provide a variety of resources related to the site's purpose. You should also include a variety of links, including links to sites that are offering backlinks to your site. If related sites link back to your site, your site ranking will move up in the rankings on search results pages.
Spam
Watch over your site so that you can get rid of spam right away. This is imperative for forum and blog administrators. A professional website designer will always be prepared to wipe away the trash and clean up the page.
As you've read, creating a quality website does not need to be that hard. You will need to understand the basic fundamentals though. Just use the information you learned here.