CodeFest Manthan 2011 was my first competition on Codeforces, and I enjoyed it a lot.  Thank you very much to the organizers of both Codeforces and CodeFest.

Here are two feature requests on the system of Codeforces from the viewpoint of a newcomer.

First, the most important request is: let us practice locking a problem and hacking solutions!  (If it is possible with the current system, please let me know.)  Hacking competitors’ solutions is an essential part of the competition system of Codeforces, and it is important to learn and practice the steps to do it.  Ideally, it should work during practice just as in real contests except that (a) one solution can be hacked successfully more than once and (b) a user can “unlock” a problem.

The second request is less important than the first, but: the test cases used in the past contests can be presented better.  In particular, the test cases submitted in both successful and unsuccessful hacking attempts should be available (after contests, of course) because they will be valuable information to learn how to create good test cases for hacking.   (Edited: see below.)  In addition, some test cases in the system test are not shown in full; for example, try opening any correct solution to Problem A in Manthan 2011 and look at Test #6.

Hope this helps.

Edit: I learned that the inputs in hacking attempts in past contests are available on the website: choose Contests → (choose the Enter link for a contest) → Hacks.