Great questions Matthew!
Page-level SEO is centered around relevancy, trust and authority. The only way dynamic elements might hurt you is if you are linking to inappropriate content, over-linking outside of the domain, or producing heavy load issues with a giant, unoptimized, image or other media. The Panda Update has been said to have dinged web pages packed with ads as well. If the experience is slow or spammy, typically the user will return to Google and a choose a competitor, which could be interpreted as "I didn't find what I was looking for on this site."
Yes, Google does look absolutely look at your XML Sitemap's Last Updated information, but not as a ranking factor, just as a "please revisit and refresh your cache" type of activity. There's also a Revisit-After meta tag that can be added at the page level, but it's not recommended and at times has produced the opposite effect.
My suggestion in terms of page-level optimization is not think too much about SEO (aside from keyword prominence in title, meta data, headings and maybe once or twice within the body of the content).
Instead, think about what the user really wants and then over-exceed their expectations. Example query "blue widgets". The title might say "Buy Blue Widgets Half-Price Until 1/2/12 - Limited Supply". The meta might say "Joe's Blue Widgets use state-of-the-art technology, have won 6 awards and been featured on ABC."
Find Bowflex in a search for "home gym". Compare their landing page to that of a competitor to see that they absolutely over-deliver to their visitor, making it less-likely that a user will return to Google to choose a different result.
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