SEO depends on Google as much as it’s opposed to it. For every new SEO technique, there is an eventual algorithm update to stamp it out. As such, many of the tricks that used to work in the early days are now at best ineffective, and at worst will get you penalised.
Artificial link profiling
High numbers of keyword rich links lead to higher rankings (e.g. you might rank highly for light bulbs if a thousand sites link to yours with the phrase light bulbs) but overdo the same keyword and you’ll catch Google’s attention.
Penalisation is yours for either running or using them. If your site is content-light and link-heavy, don’t expect to get anywhere soon. If you have a lot of links from link farms, expect the same.
The opposite of having too many outbound links. Of course, you’re not obligated to link to other sites, but recent Google announcements have implied that overly-judicious use of the “rel=nofollow” attribute (formerly good SEO practice) is frowned upon.
Large link packages
The 1000 links for $10 sorts of packages you find on disreputable SEO forums. You might get a short-term boost, but chances are those links are on unrelated, link-heavy sites that are all connected (and therefore discoverable algorithmically).
Google will flag you for accumulating lots of links very fast, probably from link farms, and possibly with the same anchor text. Generally a bad idea then.
Either duplicates of your own content (i.e. two pages the same) or copying other peoples content. After the recent Panda update, Google is particularly hot on content scraping sites that re-write other peoples content for their own.
High instances of keywords used to guarantee you a top spot in the SERPs. Ensuring a decent keyword count is still important, but hidden lists at the bottom of the page are not the way to do it. If you feel like youre overdoing it, you are.
Non-HTML site elements
All SEO is about gaming the system to a certain extent, but if it feels like cheating, just think how Google would view it.