CDFs could be radically improved by reimplementing them as standard View Fields. Currently, the user must navigate away from Views in order to configure a CDF and therefore loses the current Views context. Instead, a generic Views Calc Field (VCF) could always be available from Views.
When added to the View, the VCF would provide the same text area for building the SQL query as before. The advantage would be that the list of tokens could be filtered down to those available from the current View. Even Views that require arguments would not be a problem. Using dummy argument values would still allow a list of SQL tokens to be extracted from the View.
Furthermore, the SQL calculation created by the user can be validated against the currently available tokens, simplyfing debugging. I suspect this could also be a way for VCFs to adopt support for relationships by relying on the Views context rather than a generic list of CCK fields, et al.
While the current CDFs are available to all Views, it does not seem that useful as the calculation is often specific to fields being used in the View. If code-reuse is of interest, then an API à la Computed Fields could be the solution.