In my 10th grade classes, we are using standards based grading, and so far, it's supporting the goals that I have for my classes immensely because the grades are composed of both mastery of learning objectives and the "softer" practices we also want to foster in students. Grades are seen as individual pathways and ways to continue improving and to get more focused coaching from teachers on how to get there.
In my 7th grade classes, for which I don't give grades, only narrative feedback, I am really struggling with how to focus students and have them work at getting better without the structure and clear communication imposed by a grading system. For example, I gave a quiz a few days ago and there were a number of students who didn't demonstrate mastery on a few topics. Today, I gave the quizzes back with lots of feedback. I also made a spreadsheet like this for each student, with an assessment of mastery on each topic and very specific comments as to what they should be working on:
Quizzes were given back and students were told to rework problems on the quiz that they got wrong, first asking their group for help if they were stuck and then me if the whole group was stuck on the same question. They were also given extra practice problems for each learning objective. The result was pretty crappy. They were not engaged with this at all. Instead of helping students or groups with questions on which they were stuck, as I imagined I would be doing, I spent my time policing a class of students who wanted to do anything in the world but the task at hand and putting out behavior fires.
My middle school classes do well with open tasks, interesting projects, games, and puzzles and I totally believe that we should have lots of those things in a Math class. But, I also believe that students need to be able to demonstrate understanding of course objectives. Without grades, I don't know how to build in accountability for doing the latter. If you're 12 and don't really care if you can't set up and solve percent problems, how do I make you care? Is a class supposed to be full of fun and rich activities at all times?