I think it is always possible to say something positive about another's work.
"Your quilt is a large one...so much work"
"I can see that you like brown"
"I'm not familiar with that pattern. Did you design your quilt?"
"I like small projects too"
"I love seeing everyone's work"
There are many things that can be said that are not critical. I must admit though, that if someone made the comment that my work was "interesting" I would perceive that as..they hate it.
I also would never give them unsolicited advice on improving their skills, or color choices. I think that could be interpreted as critical and somewhat "quilt police" like.