> The reason Kepler ran out of fuel when it did was that it was expending more maneuvering fuel for attitude control because two of its four reaction wheels failed. Hubble and the ISS also had similar failures.

And to calm down anyone afraid of JWST sharing the same fate - construction of reaction wheels have been changed some time ago to make them significantly more reliable. The source of issues on Hubble, Kepler, FUSE, Hayabusa, Dawn and TIMED was electrical arcing between metal parts of reaction wheels. Static charge was building up like when you rub a ballon against your head. That charge caused arcing that in turn caused metal pitting and increased friction leading to failures. That failure mode was understood only in late 2007, when Kepler was already fully build and ready for launch.

JWSt uses new generation ceramic bearing in its reaction wheels, they have been used in spacecrafts since 2010 with great performance.