Shuttle polar orbit
The reason the Shuttle never flew out of Vandenberg was because Challenger exploded.
The Air Force had already been getting cold feet over the Shuttle's poor record of getting into space on schedule, Challenger was the final nail in the coffin.
As you said, polar orbits required extra oomph! to get into space; the USAF would have required ultra-light SRBs to make this possible. These would have been even more prone to joint failure than those used on Challenger, so there was no way the programme could continue. Instead the USAF went back to Titan IV rockets which came with their own set of problems:
http://www.youtube.com/watch?v=nqlgUuYQU30&feature=related
Of course, if the USAF hadn't insisted on cross-range capability for the Shuttle, then NASA could have had a simpler, lighter, less fragile machine to do their work and the manned space programme probably wouldn't be in the mess it is now.


