Dear Roy,
As an open-source alternative to PSPP, there is a hard-working community adding new features to PSPP, but it does not, and may never match the full set of capabilities of commercial SPSS. Rather than list what SPSS does not have, it may be more productive to look at the list of what tasks PSPP can accomplish to see whether those tasks will meet your needs. As a university lecturer, I find that PSPP can complete most anything students might need during their first year of social statistics classes (cross-tabs, t-tests, ANOVAs, regression, logistic regression, data management tasks, etc).
Renan