I have noticed recently that in the tags, beginner, intermediate and advanced are all missing. Is there a reason for this? 

They were intended to be used on tutorial posts so that people could easily find tutorials based on their skill level, but instead the tags were abused for unrelated things that made the original intent useless... for example, people often posted crash reports with the "beginner" tag, so instead of finding tutorials for beginners, you got crash reports showing up in searches.