Strong visual capabilities, drill down function and data browsing. No need for any other tools, ETL/ scripting and visualisation built right into one tool.
While Qlik and their products are very intuitive and a pleasure to use as a developer of the dashboards and poweruser, many of my colleagues and stakeholders of information are used to static (paper) reports. This often leads to producing tables and graphs that are 'pinned down' (set analysis) to reduce the flexibility of the representations and the dashboards as a whole. (Qlik presents this as a core strength of their products.)
Often this makes the formulas for these expressions and dimensions very complex. Of course you could argue that the users would have to 'mature' to use this new, more flexible, way of information. But it would be a great improvement for the Qlik products if it were able to easily 'lock' graphs without extensive coding.
Due to in memory technology, when datasets get really huge, rendering of graphs can take some time or even return out of memory.
QlikView is a great BI product. You do not need any other tools to get your information from your systems (even with addition of extra data sources) into nice graphical representations.
Even though QlikView has printing capabilities and you are able to create reports, this is not the best tool to create static reports. The core strength lies in the ability to browse through the information in the dashboard and have many questions answered within 5 minutes.