Google SEO

"Like a mother Grizzly watching a hunter poking her cub with a stick."

Legend has it that was how one of The Founders viewed SEOs which is quite understandable as they were made to look foolish by a bunch of self-serving opportunists, and despite the recent Glasnost I suspect this opinion hasn't changed and still underpins Googles' attempts at combating manipulation, seen most obviously these days on the "Linkage Front".

Historically Google has never been able to properly "police" its' guidelines, so those that complied with them soon found themselves overtaken by those that ran a coach-and-horses through them, again most notably in the linkage sector where a veritable links industry has developed, from auction houses specialising in the sale of text links to networks of "paid to blog" merchants.

This all makes optimising for Google a bit tricky, you can have top rankings or you can have safe rankings, but to obtain both simultaneously calls for a patience and subtlety largely missing from the SEO sector.

Spam and "Intent"

A word we're increasingly coming across in relation to how/what G determines as Spam is "intent".

This means that rather than jumping in at the first sign of a guideline violation that could just be accidental, G gives us sufficient rope as to dispel any doubt as to the intention behind the violations.
Note the plural, in just about every linkage-based penalty I've seen there invariably were multiple violations in play.

Spam Penalties

Unsurprisingly most of the penalties we see are for link based violations, some years back they'd get a PR0/ban so were fairly easily identifiable, but these days those type of penalties are much rarer, what we mostly see are ranking drops of varying severity making identifying the cause of the penalty somewhat more difficult.

In conclusion play SEO with Google carefully, initially it can appear quite forgiving which can lead to over-confidence, and greed is the single greatest cause for webmasters passing that tipping point that eliminates any doubt about intent.