Great question.
Several years ago I would of said a good piece of furniture was good when all the joints and construction was perfect.
These days I would say its any piece that make me look twice, makes me want to touch it and really gets me thinking of how they did that. I really like when someone has taken a piece of timber and built a piece because of the wood rather than just following a plan.
I remember seeing a lovely table at a show where the lady had taken two slabs of Oak and had made a waney edge table for eating outside by book matching the two pieces, the top was full of character with swirls and little knots. She was originally going to make everything square and straight but the wood just spoke to her, and she had used a scrub plane to get things level but liked the scallops it made and left it like that. She then joined the two pieces with big butterfly keys, which she said because of leaving the wood as slabs and being outside, movement was going to happen so why not let it be part of the table and encompass it rather than fight it. To some this piece would of been overlooked but to me she had got the best out of the wood and made a perfect table.