1. The most common infraction is keyword stuffing/spamming/abusing etc. When desperate for rankings people will often resort to just repeating keywords as often as they can. You need to focus on proper keyword usage and the best test for this is to read your content out loud. You’ll know if it sounds keyword stuffed and you can go back and edit it. One way to avoid keyword stuffing is to use different variations of your words and phrases and look for synonyms that you can get in there. That eliminates the over usage of one phrase and it creates more phrases you can possibly be found for.
2. Lots of low quality links all with the same or very similar anchor text. This dates back to the days of the good old fashioned link exchange – “you link to me and I’ll link to you and by the way, let’s both have everyone linking to us use the same anchor text and all link to only the homepage”. Not cool these days. As with most things in SEO, it’s about balance and well rounded diversity. You want quality links to many different pages, all with different anchor text (that is relevant to the page you are linking to). These kind of links are hard to obtain but Google doesn’t care. It’s what they want. Most people want a quick easy link fix and it was never a good idea but it’s even more important that you avoid it these days.
3. Too little content on the page but lots of “optimization”
4. Lots of duplicate content taken from sites that rank well.
5. There is some theory that too many sites being pointed/redirected to your main site could hurt you. I’ve heard a lot of talk about this one but not seen a lot of proof
6. Link farms/networks. There are two ways this can hurt you. If you are running a link farm – which means too many links on a page, none with any real text or purpose other than giving out links. The second way is having a link farm/network linking to your site (especially if the same ones have multiple links back to your site).
over optimization has always been a bad thing. It’s been called spamming and black hat (which also involves cloaking and various other nefarious things) but the bottom line is Google has been continually ramping up their efforts to weed out bad sites.