I've heard that its a bad idea to create passwords based on a common form such as random letters appended with the start of a websites domain name. So for example a password to gmail might be something like 8*dfJK3!%kgmail and a password to facebook would be 8*dfJK3!%kfacebook and so on. Why this considered a bad idea?
Reusing passwords across multiple sites is very problematic as a password exposure on one site leads to a password exposure on all of your accounts that used the same password. What you are proposing seems better than using a static password across multiple sites, but it is still rather predictable. If someone discovers one of your passwords they can likely guess many of your other passwords. No matter how careful you are with your passwords, they will be discovered. Sites get hacked and password lists are published.
The only way to ensure password safety is to use unique random passwords that have a provably large entropy (a measure of randomness). This answer discusses randomness as related to password generation.
Because it's easily predictable. There was a study done on organizations that enforced a password policy which required every password to contain at least 1 symbol and 1 digit. The results of the study concluded that passwords of the form
were very prevalent. A malicious attacker can use this to his/her advantage when cracking passwords, drastically increasing the success rate of finding a match.