Most of the new domainers including myself usually have this question that which letters are to be considered as premium ones and which are to be avoided at any cost. After searching on Google and reading plethora of information regarding this questions, i came to a conclusion that there is no specific rule defining the importance of the character in a domain name. it entirely depend on the domain name in question. If it is a popular keyword and contains some non premium characters than there is no harm in using that domain as your website. In fact that particular domain will help in SEO of your future website.
But still there are some English alphabets which are considered as premium and non premium. They are listed below:
Letters which can be used in domains without restriction: 
Letters which are to be avoided but can be used if needed: 
Letters which to be avoided always: 