It is easy only to talk in abstract terms. Don't be a dick -- what does it mean? Nobody knows, unless they do. Some well intended people ended up in very bad sitatuations while always thinking they were not dicks.
Most people familiar with the phrase “good Samaritan” or “going the extra mile” won’t demonstrate such behaviour.
Instead, although “nice” almost certainly has some culturally specific Shibboleths, I would say the basic test is where someone is on the selfish-empathetic scale, and further to empathic is a good thing in a partner.