I mentally gave up on LAPACK as an option a long time ago. The FLAME library has more potential, it is LGPL'ed and faster than LAPACK, but it does not have all the functionality [1].
Realistically I see the role of GSL as an alternative to Numerical Recipes and other similar non-free libraries. None of these have any super features but they are still widely used. As such, I would see simplicity, ease of use and good documentation as priorities. [1] http://www.cs.utexas.edu/users/flame/
