Almost 50 question and answer sharing sites are identified on the Systems for Sharing Problems home page. It is unlikely that with this diversity of interests and systems any single site will match the features of every other system. What is more important is to have an awareness of the range of features across these systems, and have priorities about which features are most important. My interests are in long term community building, accompanied by the knowledge that the most interesting problems will be contributed to by many over a long period of time. The key questions of professional fields might be researched for decades. Consequently, unlimited time to leave an answer or response to a question is a key factor in determining the ranking on the Systems for Sharing Problems page. Such a feature then by its nature makes other features, such as point systems, and lists of most active and effective contributors and responders somewhat problematic.
My explorations of all these systems are still at an early stage, but the Yedda site has stood out by its unlimited time to respond design of their web site. That is why it is currently at the top of the list of free systems for sharing Still Unsolved Problems (SUP).
Further comparisons will depend on more detailed and accurate information about the features of the different systems. That work is underway. A comparison table of these question focused systems using a Google Documents Spreadsheet is in development. If you are interested in becoming a contributor to this spreadsheet please email me requesting access. Send requests to houghton at email.wcu.edu. If you have the data on a specific system or systems, I can send you a form page to complete that will add the information as a new row in the spreadsheet. If other columns need insertion in the spreadsheet so as to create new feature concepts, please suggest them.
There is much about each system that the company web site 'help' and 'about' pages do not explain or tell. Verifying the accuracy of such information requires data sources that can be checked. As this table fills in, it will be easier for users of such sites to select the one best for their needs based on the features that are available.