Here’s a #Moodlewish from Julian Ridden (@Moodleman) that I am all about. You can have conditional availability based on grades and student activities, but in order to differentiate assignments you need to use groups and groupings (a more complicated feature). As Julian explains, the placement of the option is more intuitive and handy (and generally will add functionality in a usable way).
Check out the idea here: http://moodle.org/mod/forum/discuss.php?d=202512
Vote it up in the tracker here: http://tracker.moodle.org/browse/MDL-30554