The quality of the links you have coming in to your pages depends on the quality of the site they come from. If your links are coming from trusted leaders in the field, as well as .gov and .edu sites that have at least an inkling of relevancy to your topic, then you are linking well. Links from quality websites to you (assuming the linking page itself has good trust and authority) can indicate that you’re on the same level but don’t necessarily give you the same authority.
When you throw in the url significance, that also has an effect. You want to be linked to from a url that has some impact – not some sort of session ID or randomly generated url. Some SEOs claim that a .PDF or .doc extension can affect weight quality – this seems likely only if the doc itself has numerous inbound links giving it authority of its own.
Links from pages which update frequently may or may not be of higher value – a static page with links to it being added over a long period of time is valuable as it is not outdated, but at the same time a news site which is updated daily may be a high quality link since they are being kept up to date. It depends on the type of site.
Negative links, of course, are any (excepting directory) links which are paid or sponsored and not no-followed. You want to buy links, fine – just don’t expect PageRank along with them and make darn sure that Google doesn’t spider them as the penalties for using paid links to artificially boost your site’s rankings are huge.
However, if you are running around trying to get links from any and everywhere, make sure you aren’t getting them from a no-follow source or non-indexed page as this won’t help you at all and you may just be spinning your wheels SEO wise. Of course, for traffic purposes, any link is a good link.
Remember the basics – link responsibly, link relevantly, and link routinely. The more high quality links you have, the more your site becomes an authority in its own right and the closer you are to the top of the pile.