I use KNIME to do fairly large transformations across a range of data sources. Testing data at certain checkpoints can become quite cumbersome. I propose that some form of widget(s) is provided that allows users to see the data as it runs through the system. Maybe a basic widget or text display that visualizes basic things like totals, number of rows, flow variables or even logical assertions (e.g. true, false) and tests to make sure data integrity is maintained throughout a sophisticated ETL. This would really speed up debugging by allowing some of the assertions to be tested as attributes get transformed from source to target.
Just a thought - perhaps something that someone else would also be interested in?