SEO has come a long way since its murky early days of link farms and obvious keyword stuffing. Not only are these practices unsightly and suspicious looking to potential customers, but now they will also get you into heaps of trouble with Google. The Penguin update that's been causing site owners to nervously check their analytics has changed the way we view content. While keyword stuffing has been a bad idea for a long time, it's now a serious offense.
Right now you're probably thinking "Okay, keyword stuffing is bad. But how do I know if I'm guilty of it?"
Don't be stressed. Take a look at your site and read it over. Does it look like a human being wrote it? Does it read naturally? Do you avoid squeezing phrases like "best dentist Chicago" into every third sentence because that's what you want to rank for? Chances are you're fine. If we learn nothing else from these big Google updates, it should be that readers should always come before the search engine crawlers.
As long as your keyword appears naturally in your content, repetition is not a bad thing. For example, if you're trying to get your page for Vitamix blenders to rank and you have the word Vitamix appear regularly throughout the content, that isn't a problem. After all, how can you talk about what you're selling if you never actually say its name? It's when you try to squeeze in unnatural sounding phrases like "cheapest price Vitamix blender" that Google may take notice.
Content should always be well written, engaging, and useful. If the future of SEO is sharing and social media, then it only makes sense to focus our efforts on the people who will be doing our advertising for us through Twitter and Facebook: our readers.