>Selecting Hadoop before you have a specific and concrete need for it--or see that need approaching rapidly on the horizon

the great achievement of Hadoop isn't performance, throughput, etc... (and in many cases it can actually be worse than alternatives).

The great achievement is that it is an easy (i.e. on the orders of magnitude) accessible (i.e. easy installable, configurable, scalable, supportable, programmable) cluster for "mere mortal" enterprise. Before Hadoop, any clusterable software (where even laughable by Hadoop standards 2 nodes were already priced at the cluster level) would cost arm and leg and 4 or 8 nodes would be considered a huge cluster. Hadoop gives a chance to an enterprise (not to CERN :) to taste an unconstrained (loose use of term) distributed computing, and the next generation of software is already coming - Impala, etc...

And obligatory - "nobody gets fired for using Hadoop".