The International Mathematical Union has recently released a report about the uses and misuses of bibliometrics. Applying these metrics (e.g., impact factors) to rank papers, journals, and individual scientists is problematic at best. At worst, it's misleading.
In addition to making these points quite convincingly, the report includes some fascinating graphs. Consider, for example, the graph of the average number of citations per article for various disciplines. What do you think this implies for impact factors?