I believe a student's grade should be based upon what they learn, not what they do. Effort, participation, timeliness, attitude, and behavior are important aspects to learning, but they are difficult to measure and in the long run do not accurately portray the student's level of mastery over the subject matter.  Only assessment of knowledge can be used to judge a student's--well--level of knowledge.  

Also, some students need very little time to learn and some need more time and extra chances.  Therefore, I allow students as much time as they need to master the concepts of the class. This may mean something FINALLY clicks in November that was introduced in September, for example.  That student would earn the same score on that concept as a student who got it the first time. This drastically reduces the likelihood of a student "giving up" just because their grade is currently "in the toilet," as it were. I have seen students with low F's finally learn--well--how to learn and turn things around, passing the class with a C or better!  Watching this happen is one of the biggest reasons I am in education!