Is forced or mandatory participation in 360 degree feedback a problem?
360 degree feedback (or multi-rater feedback) works best when the receivers and givers of the feedback participate voluntarily, and can choose who they wish to give feedback to, and choose the people they wish to receive feedback from.
Since the point of 360 feedback is almost entirely based on the idea that feedback will improve performance we need to acknowledge that feedback that is unwanted, or forced upon the recipient is just as likely to anger the person as it is to be received with an open mind. Simply, unsolicited (or forced) feedback is not likely to result in what we want, which is better performance.
Consider also how coercing people into giving "feedback" to co-workers, or a boss may affect the quality of the feedback. Coercion doesn't set a proper tone or organizational culture to foster the kind of atmosphere needed to make 360 degree feedback work. Multi-rater feedback really requires a degree of trust, and a culture of cooperation.