Why Yahoo Answers is bad?

Because best answers are chosen by one person they sometimes get chosen incorrectly. This can give people misleading information in some cases.

It's not unusual on Yahoo answers to find a troll or a bully making fun of someone or making a Racist remark.

The fact that a question can be closed after sometime makes the answers of some questions not comprehensive.

The partial anonymity of users , who register by any name and email, can sometimes prevent people from writing good answers as they don't care about their reputation much.

The Yahoo answers interface is cluttered and can sometimes be distracting. (See Why Google Answers failed?)

In many occasions questions that are considered of a lower quality can be found on Yahoo answers.

It's not uncommon to see a sarcastic comment getting so many up-votes in one question. Because the Community has a lot of bullies this problem shows often. 

Some people believe that Quora attracted many of the intellectuals leaving less educated people on Yahoo answers. (See Why is Quora better than Yahoo Answers?)

Because wrong answers don't get revised they usually remain on Yahoo answers unlike Wikipedia where pages are usually checked more than once.

According to an argument by some Quora users: The shallow questions on Yahoo answers attract people who later on post shallow answers to Yahoo answers.

Unlike Quora and Wikipedia Yahoo answers have no real community. Instead the site has many registered users who don't have some of the properties of a real community.