And who's supposed to judge the "difficulty rating" of a puzzle?
If it's the original poster of the question, that's almost certainly going to be subject to lots of bias. "Oh, I don't want beginners to see my question and annoy me with too-easy responses, so I'll just rate myself '5 stars.'" "I made this puzzle and I think it's really good, so I'll use 5 stars." "I know the answer to this puzzle; therefore, it must be very easy." (You know those puzzles where the answer is really obvious when you see it?) Etc.
A "community-judged difficulty" system has a number of problems as well. What happens before a difficulty has been decided on? How do you decide the difficulty? How do the votes work; is it just "hard" or "easy"? Or if it's 5 stars, how does that work? If the people rating it haven't solved the puzzle, how are they supposed to judge its difficulty? And when they have solved the puzzle, again, the answer seems really obvious so it must be "1 star," right?
Regardless of who chooses the rating, "difficulty" is still extremely subjective. The simple fact is that a really easy 15-minute puzzle for Person A might take hours to solve for Person B, but Person B is excellent at solving, say, Rebus puzzles, and he can breeze through those while they're extremely difficult from Person A's point of view.
Oh, and for the record, tags are an absolutely terrible system for this kind of situation. A "beginner" tag would be considered a meta tag, and from experience, those almost never work out in a beneficial way for the community. (I'm not going into more detail about this because the linked blog post explains it better than I ever could.)