Factors that determine a quality link in link building?

What are things to look out for when considering the value of a link? What factors make a back link good or bad?

The factors seem to change drastically every year. Here are some things that were considered good but are now classified as black hat by Google:

• Guest blogging
• Side wide links

Which things are good (and white hat) today?

What you refer to is secondary quality metrics, not primary. Primary metric has always been the same - PR/Moz/URL/Domain rank from various tools - which tells you what kind of SEO juice the links are passing. The URL rank is most important because the rank is passed to your site/URL.

What you are referring to as "black hat" are actually perfectly legitimate tactics and are still valuable, as long as they are not overused. Don't confuse these with things that are explicitly prohibited by Google.

Google penalties are for improper or aggressive use of a method. If your link building methods are balanced out and your link profile appears natural, you don't need to worry about Google penalties for certain tactics.

Relevancy: is the site's content relevant to yours? If your site is about furniture, a link from a blog about cars will be worth far less than a link from a site about furniture.

Placement: contextual link within an article are worth more than a link in the sidebar or footer. Another thing to avoid is sitewide links.

Trust Rank (metric from Majestic SEO's tool): are the links pointing to the site that links to you are in "good neighborhoods"? i.e. The site that links to you must not have spammy links itself.