There is a feature to see all Div1/2 X problem on A2OJ altogether. Also, we can find all dp problems on codeforces using tags but how can I see all Div2 C problems which include dynamic programming? Is there some way? If it's not, I would request this feature and I think it will help many people.
The current feature in CF allows us to sort problems by number of users and that IMHO, is a better indicator of difficulty measure of a problem than stating that the problem was a Div2-C or Div2-D.
As an aside you can even fine tune the search results to a particular set of problem tags by separating the tags by commas in the URL. Example.