The recent incident where attackers posted usernames and passwords for compromised Dropbox accounts really shows the importance of practicing good password hygiene.
GigaOm has one of many articles describing the actual events. The short version is that some hackers have been posting usernames and passwords to Dropbox accounts on a Pastebin page. Dropbox says that they have not been compromised, and that the passwords were actually taken from other websites or through other methods.
If this is true, and it seems reasonable, then those who have been compromised became victims because they reused their passwords across multiple websites. That is probably a bigger security error than choosing weak passwords in the first place.
The security at websites varies widely, usually based on the sensitivity of the information on that site. Banks tend to have better security than news sites or discussion sites. If you use the same password with all these sites, then if any of them is compromised the attacker can simply try your username / password on every other interesting website to see if they work there too.
The solution is to use a different password on every website. They should not be simply modifications of each other but actually completely different passwords. Additionally they should be long and random. This means that they will be impossible to remember, but a password manager or password vault can take care of that for you. It will generate the strong random passwords, fill in the forms for you, and sync between your various computers and other devices. There is no excuse not to use unique and strong passwords with every website, and you will be much safer if you do.