From the Google Webmaster Central Blog

Why should you upload a Sitemap to Google Webmaster Tools, if you site is crawled just fine?

All sites can benefit from submitting a Sitemap to Google Webmaster Tools. You may help them to do a better job of crawling and understanding your site, especially if it has dynamic content or a complicated architecture.

Besides, you will have access to more information about your site, for example the number of pages from your Sitemaps that are indexed by Google, any errors Google found with your Sitemap, as well as warnings about potential problems. Also, you can submit specialized Sitemaps for certain types of content including Video, Mobile, News and Code.
More information about the benefits of submitting a Sitemap to Google Webmaster Tools can be found here.

How do you detect paid links? If I want to stay on the safe side, should I use the "nofollow" attribute on all links?

GWC blogged about our position on paid links and the use of nofollow a few months ago. You may also find it interesting to read this thread in our Help Group about appropriate uses of the nofollow attribute.

How do I associate my site with a particular country/region using Google Webmaster Tools? Can I do this for a dynamic website?

The instructions in the Help Center explain that you can associate a country or region to an entire domain, individual subdomains or subdirectories. A quick tip: if, for instance, you are targeting the UK market, better ways of structuring your site would be example.co.uk, uk.example.com, or example.com/uk/. Google can geolocate all of those patterns.

If your domain name has no regional significance, such as www.example.com, you can still associate your website with a country or region. To do that you will need to verify the domain, or the subdomains and/or subdirectories one by one in your Webmaster Tools account and then associate each of them with a country/region. However, for the moment Google does not support setting a geographical target for patterns that can't be verified such as, for example, www.example.com/?region=countrycode.

I have a news site and it is not entirely crawled. Why? Other crawlers had no problem crawling us...

First off, make sure that nothing prevents us from crawling your news site - the architecture of your site or the robots.txt file. Also, we suggest you sign up for Webmaster Tools and submit your content. We specifically have the News Sitemap protocol for sites offering this type of content. If you take advantage of this feature, Google can give you more information on which URLs we had trouble with and why. It really rocks!