How to decide priority of the feature?

Greetings everyone,

From long been, I was thinking about few questions. For some one point of view, it might look very stupid and novice.

However, I thought to share with you all.

Here we go,

1) How product manager should cut the line between setting up the priority of the features? (ex. Line between MUST HAVE or SHOULD HAVE or NICE TO HAVE)? Now the catch is, while we develop MRD/PRD; few features have been recommended by customer, few from services, few are from engineering, few may be very newer and innovative ….how are we going to decide priority for each of these? and would set “X” priority?  and If “X” then why only “X” and not “Y”? (I know, its more situational and depends on customer demand and market conditions, but still is their any basics we can follow!)

2) Is their any generic strategy/criteria (Other than ROI), which we could apply while deciding up the priorities?

3) How are we handling/looking at the ROI of any feature we recommend as MUST HAVE?
4) And most importantly, What if feature we perceived as MUST HAVE is not being used by customer after actual release of product?
It indirectly means, our perception about feature was wrong! How to handle these differences?

Anyone would like to put up some thoughts on these !

Thanks,

-Yogesh