Your place in the search results (your search ranking) reflects your online reputation. If your site shows up at the first page (or most preferably, is in the first three ranks), your site is considered by the search engine to be the most authoritative and most relevant site, out of all the millions of sites being indexed. Online reputation is very hard to build up, but really easy to destroy. Here are some things to be on guard for.

Spam comments
Many of the major search engines will give penalties if your website contain spam keywords. While you might not be using them when you are creating the content of your site, blogs and forums often have a comment facility that allows other people (spammers included) to add content.

Spam comments should be avoided not just because of search engine penalties, but because it portrays your site as unprofessional. It also detracts from the reader experience (nobody likes to read spam something spammers really need to learn). Add a captcha or moderate your comments to prevent this.

Doorway pages
Doorway pages are pages whose only function is to redirect a reader to a different page (usually not at all connected to the content of the doorway page). For example, a doorway page might contain a lot of words for gardening that a searcher might put as keywords; however, the page itself does not contain any relevant content on gardening and will only link to some insurance website.

Doorway pages might have been very effective (tho
ugh still not very useful) back in the early days of search engines, when their algorithms were simplistic and simply used page contents as the sole influencer of page rank. Search engines have evolved in the meantime, and these types of pages are frowned upon.

Hidden text and links
Another holdover from the early days of search engines, hidden text and links are those that have a foreground color that is the same as the background color. The idea (albeit misguided) here is that search engines only see text from a web page, and pays no attention to color. Therefore, only search engines will see the hidden text and links - people with browsers will not see them (and will probably think the site is not spamming).

Of course, search engines have evolved since then, and will give penalties to a site that uses such techniques.

Malware on your site
Having malware on your site will often send a red flag to search engines and will either give a penalty to your site's ranking, or will cause a warning page to pop up before the searcher is directed to your site. Nonetheless, having malware on your site detracts from the user experience - you might lead the user the first time, but he is very much unlikely to return.

You might not be (hopefully) installing malware on your site deliberately, but there are still ways to have it on your site inadvertently. If you are running a blog or a forum, you should update your software often - sometimes these software will contain vulnerabilities that allow people with questionable intent to take control of your site and attach malware to your pages.