There are a few things I feel could be tweaked. The number one would be allowing users to create more than 6 labels. It has been asked for a number of times on their uservoice page and always gets the response "we want to keep it simple for newbs".... I don't understand how allowing "expert" users some way of creating more labels impacts on newbs......
Most other features which I feel trello could do with have been plugged up by the amazing trello plus chrome extension. Trello should probably just hire the guy who makes it.