Default passwords are bad. Many systems will have a default password that many people never bother to change. Which of course means that someone who knows that default can easily gain access to a number of accounts! For one extreme example, consider a story from Richard Feynman's autobiography, Surely You're Joking, My Feynman! A military big-shot is not available and some papers from his safe are needed; unfortunately, nobody knows the big shot's safe combination and they're unable to reach him. However, someone on the base happens to know the default combination on this huge, heavy, very expensive safe loaded with highly classified documents... and it turns out that the big shot never bothered to change it. Oops! Don't make that mistake; whether it's a wireless router or something else, if you have something with a default password and you care whether people can get in, change the password!
Hackers will commonly find passwords by simply trying every word in the dictionary; this makes dictionary words a bad idea! Particularly popular (and thus particularly bad) choices include god, love, lust, money, password, private, qwerty, secret, sex, and snoopy, but any dictionary word is a bad choice.
Other bad passwords are those that can be easily guessed, such as your zip code, birthday, kid's names, spouse's birthday, etc. While these have the advantage of being easy to remember (see below), they're also easy for someone else to guess.