What is the lowest common denominator of cancer?
Hanahan and Weinberg's "Hallmarks of Cancer" articles should answer your question.

Their original, highly cited (14k+ citations), [Six] Hallmarks of Cancer article list these six common attributes of all cancers:


Sustaining proliferative signaling
Evading growth suppressors
Activating invasion and metastasis
Enabling replicative immortality
Inducing angiogenesis
Resisting cell death


The new article, Hallmarks of Cancer: The Next Generation includes additional "emerging hallmarks" and "enabling characteristics" (Figure 3) common among cancers:


The capability to modify, or reprogram, cellular metabolism in order to most effectively support neoplastic proliferation; and
The ability to evade immunological destruction, in particular by T and B lymphocytes, macrophages, and natural killer cells.