Back when Search Engine Optimization kicked off as a profession, the first 2, 3 tricks in the bags of SEO guys revolved around On-page factors. But then off-page factors took center stage thanks to the tremendous value of links. Webmasters got so wrapped up in this link-pursuit that some stopped paying any attention to their own websites.
Day in and day out, people continued to spend time and money on acquiring back links, while their own website was left starving for attention. For years we were deaf to our website's needs and requirements, their cries for help went unheard (ok, thats a little too dramatic but, still )
And then, from the miraculous valley of silicon, arrived The Panda! Panda is no less than the knight in shining armor our websites in distress were waiting for.
Ok, enough with the sensationalism, maybe I am still high on all the reading I have done after this update. The more I read, the more I feel that Panda has pushed on-site optimization back in the game. Google does webmasters a huge favor by making them focus on their websites all over again.
Content Quality
Easier said than done, quite confusing, very complicated, rather controversial but still, quality content is something that you cannot subsist without, on the web.
There are shortcuts for accumulating content that will be mistaken for quality content by search engines for some time, but these shortcuts are short lived, The majority of sites hit by Panda were using shortcuts to generate pages with too little original or quality content.
Now webmasters have to think of novel ideas for creating content again.
Ad-to-content Ratio
Panda snubbed websites that were churning out new pages with little (or no) content and lots of ads: these pages were serving no purpose other than polluting the Internet. They were created mainly to target keywords with some makeshift "content". It was becoming a common practice to make a page for a keyword or key phrase without worrying about the content.
Now webmaster should publish new pages only when they have something substantial to offer.
Users
After Panda, it is all about the user. About staying on the site, engaging with the site, linking to the site, sharing the site. And for the first time ever Google says they're listening to what those users say where "In some high-confidence situations, we are beginning to incorporate data about the sites that users block into our algorithms"
Tricks will be figured out and discarded, so its better if you can work on improving the quality of the content, and improving the user experience.