I've inherited a legacy software system, and have been tasked with performing usability and system upgrades. While there's nothing bad with the system, from discussions with the users, there are "small" usabilty issues that need to be addressed.

At this stage I'm the lone developer on this system, and apart from testing I don't use the system at all, so its difficult for me to know what issues may exist or are percieved to exist. I'm going to have some time to speak with them all and discuss what they percieve to be good/bad or indifferent about the system.

Since its essentially just me for the time being my time is limit. So I was considering asking them to imagine that I'd only be able to do one change, have them all write privately what they'd want that one change to be, and then helping them rank those, but I'm hoping for other tips as well.

What techniques exist for getting users to explain their wants, needs, and requirements, while also having them rank them by importance or desirability?