My personal experience with qlik is limited but we did evaluate both tools for 3 months. I would say that qlik is programmer/developer friendly. If you want to do something there is probably somewhere a dialog box or wizard to let you do that. Tableau just works differently. Its visual query language allows you encode any column to many possible visualization elements. There is no pie chart wizard, no histogram wizard to step through. It allows you to encode your data to different elements and draw different mark types to quickly change visuals and see things differently. It is incredibly expressive.
A 50 year old software engineer watched me build him some charts from his tickets data once and I was going fast. Dragging, switching, changing colors, flipping, right clicking, erasing and starting over and after about a minute of this he said to me. "What the hell are you doing?", I told him "I'm trying to create the right vizql statement to meet your needs." He laughed and then said, "Tableau really is a programming language isn't it?" Yup, but not by typing text but with data driven visual cues.
